rebase: make builtin and legacy script error messages the same

rebase: make builtin and legacy script error messages the same

The conversion of the script version of rebase took messages that were
prefixed with "error:" and passed them along to die(), which adds a
"fatal:" prefix, thus resulting in messages of the form:

  fatal: error: cannot combine...

which seems redundant.  Remove the "error:" prefix from the builtin
version of rebase, and change the prefix from "error:" to "fatal:" in
the legacy script to match.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>