This allows the caller to add its own error message to that returned
by split_cmdline. Thus error output following a failed split_cmdline
can be of the form
fatal: Bad alias.test string: cmdline ends with \
rather than
error: cmdline ends with \
fatal: Bad alias.test string
Signed-off-by: Greg Brockman <gdb@mit.edu> Signed-off-by: Junio C Hamano <gitster@pobox.com>