Minor code rationalization: FlushRelationBuffers just returns void,

Minor code rationalization: FlushRelationBuffers just returns void,
rather than an error code, and does elog(ERROR) not elog(WARNING)
when it detects a problem.  All callers were simply elog(ERROR)'ing on
failure return anyway, and I find it hard to envision a caller that would
not, so we may as well simplify the callers and produce the more useful
error message directly.