Transliterator parser error messages

[skius]

Related/depends on: #3558

Decide if we want to pretty-print transliterator parser errors like we do with UnicodeSet parser errors. There are currently no transliterator parser error message tests, only general negative parser tests that assert that the returned Result is_err().

If we want nice error messages that include the source location also for the validation and compilation steps, we'd need to persist the span in the AST, maybe by wrapping each type in a Span<T>.