reflog_expire(): integrate lock_ref_sha1_basic() errors into ours
reflog_expire(): integrate lock_ref_sha1_basic() errors into ours

Now that lock_ref_sha1_basic() gives us back its error messages via a
strbuf, incorporate its error message into our error message rather
than emitting two separate error messages.

Signed-off-by: Michael Haggerty <c24c152f145c57791e809b126db9e61bd1c1782d@alum.mit.edu>

destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git,destenson/git