Add the repository and bugtracker to the metadata

Useful so MetaCPAN can link to the appropriate places automatically and
with authority.
Throw a useful error when a named field isn't in the header line

Resolves [rt.cpan.org #91199].
Improve the format of I/O errors from Text::CSV's error_diag method

What error_diag returns depends on the caller's context.  In this case,
it returned a list which then got jammed together by warn().  Instead,
format a nice error message which includes even more information (line
number, an input/output distinction).
Merge pull request #2 from tsibley/error-handling

Error handling