Use of `error`

[zudov]

Currently templatewill call anerrorif the template string is malformed, for error-handling there'stemplateSafethat returns anEither`.

I think it would be nicer if the library defined an error type (e.g. TemplateError) and template would throw that instead of an error.

[trskop]

This could be nicely combined with my proposed solution to #13.