Add Error.pm to the distribution

I have been thinking about how to do the error reporting the best
way and after scraping various overcomplicated concepts, I have
decided that by far the most elegant way is to throw Error exceptions;
the closest sane alternative is to catch the dies in Git.pm by
enclosing the calls in eval{}s and that's really _quite_ ugly.

The only "small" trouble is that Error.pm turns out sadly not to be
part of the standard distribution, and installation from CPAN is
a bother, especially if you can't install it system-wide. But since
it is very small, I've decided to just bundle it.

Signed-off-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Junio C Hamano <junkio@cox.net>