Josh suggested adding warnings to this function to help users
diagnose problems.

Other than memory allocatino errors, there are two places where
errors can be returned.  Both represent problems that should
have been caught earlier, and as such might well have been
handled with BUG_ON() calls.  But if either ever did manage to
happen, it will be reported.
Signed-off-by: NAlex Elder <elder@inktank.com>
Reviewed-by: NJosh Durgin <josh.durgin@inktank.com>