git_xmerge_config(): prefer error() to die()

git_xmerge_config(): prefer error() to die()

When parsing merge config, a few code paths die on error. It's
preferable for us to call error() here, because the resulting error
message from the config parsing code contains much more detail.

For example, before:

  fatal: unknown style 'bogus' given for 'merge.conflictstyle'

and after:

  error: unknown style 'bogus' given for 'merge.conflictstyle'
  fatal: bad config variable 'merge.conflictstyle' in file '.git/config' at line 7

Since we're touching these lines, I also marked them for translation.
There's no reason they shouldn't behave like most other config-parsing
errors.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>