git p4: catch p4 describe errors

git p4: catch p4 describe errors

Group the two calls to "p4 describe" into a new helper function,
and try to validate the p4 results.  The current behavior when p4
describe fails is to die with a python backtrace.  The new behavior
will print the full response.

This does not solve any particular problem, but adds more
checking in hopes of narrowing down odd behavior seen on
at least two occasions.

Based-on-patch-by: Matt Arsenault <arsenm2@gmail.com>
Reported-by: Arthur <a.foulon@amesys.fr>
Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>