git p4: catch p4 errors when streaming file contents

git p4: catch p4 errors when streaming file contents

Error messages that arise during the "p4 print" phase of
generating commits were silently ignored.  Catch them,
abort the fast-import, and exit.

Without this fix, the sync/clone appears to work, but files that
are inaccessible by the p4d server will still be imported to git,
although without the proper contents.  Instead the errant files
will contain a p4 error message, such as "Librarian checkout
//depot/path failed".

Signed-off-by: Pete Wyckoff <pw@padd.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>