bisect: libify handle_bad_merge_base and its dependents

Since we want to get rid of git-bisect.sh, it would be necessary to
convert those exit() calls to return statements so that errors can be
reported.

Emulate try catch in C by converting `exit(<positive-value>)` to
`return <negative-value>`. Follow POSIX conventions to return
<negative-value> to indicate error.

Update all callers to handle the error returns.

Mentored-by: Christian Couder <chriscool@tuxfamily.org>
Mentored-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Pranit Bauva <pranit.bauva@gmail.com>
Signed-off-by: Tanushree Tumane <tanushreetumane@gmail.com>
Signed-off-by: Miriam Rubio <mirucam@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

bisect.h

@@ -48,6 +48,7 @@ enum bisect_error {
 	BISECT_OK = 0,
 	BISECT_FAILED = -1,
 	BISECT_ONLY_SKIPPED_LEFT = -2,
+	BISECT_MERGE_BASE_CHECK = -3,
 	BISECT_INTERNAL_SUCCESS_MERGE_BASE = -11
 };