Change behaviour of ConfigWriter._WriteConfig

This patch changes the behaviour of _WriteConfig in case of
configuration errors:

- before, it used to abort the saving (even though the in-memory
  configuration used by current jobs has already changed)
- now, we log it (both to the log and to the user) but continue, since
  we can't revert to a good version of the config anyway

This should make the internal behaviour of the code more consistent with
the external world, even though the config might be “wrong”; we leave
the cleanup to the user. This should not be as bad as it sounds, since
we haven't actually seen this case except for the ugly master candidates
handling, and that was fixed recently by Guido's patch series.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>