Return public net/textproto error types

[dcormier]

To allow for checking returned error types, types from the standard library's net/textproto must be used instead of types from the private textproto fork.

Kind of related to #294.

[coveralls]

coverage: 86.272% (+0.06%) from 86.213%
when pulling 0378637 on dcormier:main
into ed0394e on jhillyerd:main.

[jhillyerd]

Looks good, thanks