git-p4: show detailed help when parsing options fail

git-p4: show detailed help when parsing options fail

When a user provides invalid parameters to git-p4, the program
reports the failure but does not provide the correct command syntax.

Add an exception handler to the command-line argument parser to display
the command's specific command line parameter syntax when an exception
is thrown. Rethrow the exception so the current behavior is retained.

Signed-off-by: Ben Keene <seraphire@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>