print errors from git-update-ref

...otherwise it not clear what happened when update-ref fails.

E.g., git checkout -b a/b/c HEAD would print nothing if refs/heads/a
exists and is a directory (it does return 1, so scripts checking for
return code should be ok).

I'm attaching two patches, because I'm not quite sure where it should
be done: git-checkout is the least intrusive, but only builtin-update-ref.c
has enough info to help user to resolve the problem (errno is ENOTDIR,
which is selfexplanatory). And I happen to use git-update-ref directly
sometimes.

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