gh-56634: Gettext: raise descriptive ValueErrors instead of IndexErrors for incorrect .mo files metadata

[m-aciek]

Raising ValueErrors with descriptive error messages.

• It handles empty Plural-Forms like "Plural-Forms: \n" (original downstream bug, and fix) but also lack of plural= keyword, like "Plural-Forms: nplurals=4;\n".
  • ValueError: expected ';' and 'plural=' in Plural-Forms metadata in sample.mo, got ''
  • previously IndexError
• It handles incorrect Content-Type
  • ValueError: expected 'charset=' in Content-Type metadata in sample.mo, got ''
  • previously IndexError

In the #56634 ignoring the incorrect syntax was being discussed, as well as issuing warnings.

The implementation here is following the suggestion of @serhiy-storchaka of raising an error instead.

• Issue: gettext breaks on empty plural-forms value #56634

[serhiy-storchaka]

Could you please add similar change for content-type?

[m-aciek]

• I've covered content type metadata, as requested.
• Changed error messages to:
  • ValueError: expected ';' and 'plural=' in Plural-Forms metadata in sample.mo, got ''
  • and ValueError: expected 'charset=' in Content-Type metadata in sample.mo, got ''
• Added a test for second case of plural forms and for content type error.

@serhiy-storchaka Would it be possible for you to find a spare moment for a re-review? Thank you!

[m-aciek]

(Fixed Windows support in tests.)