Add some missing EXTENSIONS sections to misc tests

Improve some TypeError and ValueError messages

Closes GH-5377

Convert warnings to ValueError in mb_strpos function family.

Closes GH-5109

Handle mb_str(r)ipos offset consistently as well

Move offset error checking into mbfl_strpos

This avoids calculating the full length only in order to validate
the offset, as mbfl_strpos needs to find the offset internally
anyway.