improve errors

[oli-obk]

Deserializing from a structure that doesn't contain a field that is required will cause the unhelpful error message about a unit being required at line 0, col 0.

While we can improve the UnitDeserializer, it would be best to first intercept the errors at https://github.com/serde-rs/xml/blob/master/src/de/mod.rs#L646 and enrich them with line and column info, because we might always run into a serde-reported error instead of a serde-xml-reported error.

Related PRs:

• don't raise a panic 'not yet implemented' for an unfinished tag #21