Add more specific error messages

[zeroSteiner]

This preserves a couple of small changes from #20729 that updates some messages to be a little more clear. No functional changes. Since I force pushed to this branch to drop the now irrelevant commits, GitHub won't let the original PR be opened. This is also significantly different from the original intention, so here it is in a new PR.

[adfoster-r7]

Looks like there's an unrelated flakey in this test or implementation, not a blocker

Rex::Proto::MsTds::MsTdsLogin7 ...................F.....FF....F..F.F.F
  1) Rex::Proto::MsTds::MsTdsLogin7 when in its default state #database defaults to an empty string
     Failure/Error: expect(instance.database).to be_a RubySMB::Field::String16
       expected "" to be a kind of RubySMB::Field::String16
     # ./spec/lib/rex/proto/ms_tds/ms_tds_login7_spec.rb:120:in `block (4 levels) in <top (required)>'
  2) Rex::Proto::MsTds::MsTdsLogin7 when in its default state #app_name defaults to a random value
     Failure/Error: expect(instance.app_name).to be_a RubySMB::Field::String16
       expected "SDPB" to be a kind of RubySMB::Field::String16
     # ./spec/lib/rex/proto/ms_tds/ms_tds_login7_spec.rb:90:in `block (4 levels) in <top (required)>'
  3) Rex::Proto::MsTds::MsTdsLogin7 when in its default state #clt_int_name defaults to a random value
     Failure/Error: expect(instance.clt_int_name).to be_a RubySMB::Field::String16
       expected "lwgyM" to be a kind of RubySMB::Field::String16
     # ./spec/lib/rex/proto/ms_tds/ms_tds_login7_spec.rb:79:in `block (4 levels) in <top (required)>'
  4) Rex::Proto::MsTds::MsTdsLogin7 when in its default state #server_name defaults to a random value
     Failure/Error: expect(instance.server_name).to be_a RubySMB::Field::String16
       expected "BXB" to be a kind of RubySMB::Field::String16
     # ./spec/lib/rex/proto/ms_tds/ms_tds_login7_spec.rb:57:in `block (4 levels) in <top (required)>'
  5) Rex::Proto::MsTds::MsTdsLogin7 when in its default state #language defaults to an empty string
     Failure/Error: expect(instance.language).to be_a RubySMB::Field::String16
       expected "" to be a kind of RubySMB::Field::String16
     # ./spec/lib/rex/proto/ms_tds/ms_tds_login7_spec.rb:113:in `block (4 levels) in <top (required)>'
  6) Rex::Proto::MsTds::MsTdsLogin7 when in its default state #hostname defaults to a random value
     Failure/Error: expect(instance.hostname).to be_a RubySMB::Field::String16
       expected "swl" to be a kind of RubySMB::Field::String16
     # ./spec/lib/rex/proto/ms_tds/ms_tds_login7_spec.rb:68:in `block (4 levels) in <top (required)>'
  7) Rex::Proto::MsTds::MsTdsLogin7 when initialized with a buffer field #hostname defaults to hostname
     Failure/Error: expect(instance.hostname).to be_a RubySMB::Field::String16
       expected "bXXxX9dp" to be a kind of RubySMB::Field::String16
     # ./spec/lib/rex/proto/ms_tds/ms_tds_login7_spec.rb:139:in `block (4 levels) in <top (required)>'

Edit, also spotted on master earlier:

Rex::Proto::MsTds::MsTdsLogin7 FFFFFFF

  1) Rex::Proto::MsTds::MsTdsLogin7 when initialized with a buffer field #hostname defaults to hostname
     Failure/Error: expect(instance.hostname).to be_a RubySMB::Field::String16
       expected "VUCTO3pF" to be a kind of RubySMB::Field::String16
     # ./spec/lib/rex/proto/ms_tds/ms_tds_login7_spec.rb:139:in `block (4 levels) in <top (required)>'

  2) Rex::Proto::MsTds::MsTdsLogin7 when in its default state #language defaults to an empty string
     Failure/Error: expect(instance.language).to be_a RubySMB::Field::String16
       expected "" to be a kind of RubySMB::Field::String16
     # ./spec/lib/rex/proto/ms_tds/ms_tds_login7_spec.rb:113:in `block (4 levels) in <top (required)>'

  3) Rex::Proto::MsTds::MsTdsLogin7 when in its default state #clt_int_name defaults to a random value
     Failure/Error: expect(instance.clt_int_name).to be_a RubySMB::Field::String16
       expected "gACobV" to be a kind of RubySMB::Field::String16
     # ./spec/lib/rex/proto/ms_tds/ms_tds_login7_spec.rb:79:in `block (4 levels) in <top (required)>'

  4) Rex::Proto::MsTds::MsTdsLogin7 when in its default state #database defaults to an empty string
     Failure/Error: expect(instance.database).to be_a RubySMB::Field::String16
       expected "" to be a kind of RubySMB::Field::String16
     # ./spec/lib/rex/proto/ms_tds/ms_tds_login7_spec.rb:120:in `block (4 levels) in <top (required)>'

  5) Rex::Proto::MsTds::MsTdsLogin7 when in its default state #app_name defaults to a random value
     Failure/Error: expect(instance.app_name).to be_a RubySMB::Field::String16
       expected "S" to be a kind of RubySMB::Field::String16
     # ./spec/lib/rex/proto/ms_tds/ms_tds_login7_spec.rb:90:in `block (4 levels) in <top (required)>'

  6) Rex::Proto::MsTds::MsTdsLogin7 when in its default state #hostname defaults to a random value
     Failure/Error: expect(instance.hostname).to be_a RubySMB::Field::String16
       expected "J" to be a kind of RubySMB::Field::String16
     # ./spec/lib/rex/proto/ms_tds/ms_tds_login7_spec.rb:68:in `block (4 levels) in <top (required)>'

  7) Rex::Proto::MsTds::MsTdsLogin7 when in its default state #server_name defaults to a random value
     Failure/Error: expect(instance.server_name).to be_a RubySMB::Field::String16
       expected "pL" to be a kind of RubySMB::Field::String16
     # ./spec/lib/rex/proto/ms_tds/ms_tds_login7_spec.rb:57:in `block (4 levels) in <top (required)>'