remote: die on config error when setting URL

remote: die on config error when setting URL

When invoking `git-remote --set-url` we do not check the return
value when writing the actual new URL to the configuration file,
pretending to the user that the configuration has been set while
it was in fact not persisted.

Fix this problem by dying early when setting the config fails.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>