bundle: detect if bundle file cannot be created

bundle: detect if bundle file cannot be created

bundle command silently died with no sign of failure if it
could not create the bundle file. (Eg.: its path resovles to a directory,
or the parent dir is sticky while file already exists and is owned
by someone else.)

Signed-off-by: Csaba Henk <redacted>
Signed-off-by: Junio C Hamano <redacted>