git update hook: gracefully error on completely broken .SRCINFO

I've seen this happen a bunch of times now. Someone cannot push to the
AUR, and the error report is some traceback with a KeyError which is
difficult to understand without context:

remote: Traceback (most recent call last):
remote:   File "/srv/http/aurweb/aur.git/hooks/update", line 33, in <module>
remote:     sys.exit(load_entry_point('aurweb==5.0.0', 'console_scripts', 'aurweb-git-update')())
remote:   File "/usr/lib/python3.9/site-packages/aurweb-5.0.0-py3.9.egg/aurweb/git/update.py", line 306, in main
remote: KeyError: 'pkgbase'

Eventually it turns out that their .SRCINFO file is... badly corrupted.
Generally, they managed to accidentally commit an *empty* file instead
of a .SRCINFO, and in all cases, the problem was on the very first
lookup for 'pkgbase'.

Point people to the actual failing commit, and have a nicely formatted
message indicating that the .SRCINFO is completely invalid.

Signed-off-by: Eli Schwartz <eschwartz@archlinux.org>