builtin/config: do not die in `get_color()`

builtin/config: do not die in `get_color()`

When trying to parse an invalid color via `get_color()` we die. We're
about to introduce another caller in a subsequent commit though that has
its own error handling, so dying is a bit drastic there. Furthermore,
the only caller that we already have right now already knows to handle
errors in other branches that don't call `get_color()`.

Convert the function to instead return an error code to improve its
flexibility.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>