git-p4: fix instantiation of CalledProcessError

CalledProcessError is an exception class from the subprocess namespace.
When raising this exception, git-p4 would instantiate CalledProcessError
objects without properly referencing the subprocess namespace causing
the script to fail.

Resolves the issue by replacing CalledProcessError with
subprocess.CalledProcessError.

Signed-off-by: Joel Holdsworth <jholdsworth@nvidia.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
