bisect: libify `exit_if_skipped_commits` to `error_if_skipped*` and its dependents

bisect: libify `exit_if_skipped_commits` to `error_if_skipped*` 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 <redacted>
Mentored-by: Johannes Schindelin <redacted>
Signed-off-by: Pranit Bauva <redacted>
Signed-off-by: Tanushree Tumane <redacted>
Signed-off-by: Miriam Rubio <redacted>
Signed-off-by: Junio C Hamano <redacted>