pgsql-server: Minor code rationalization: FlushRelationBuffers just

Log Message:
-----------
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.

Modified Files:
--------------
pgsql-server/src/backend/access/nbtree:
nbtree.c (r1.115 -> r1.116)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/nbtree/nbtree.c.diff?r1=1.115&r2=1.116)
pgsql-server/src/backend/catalog:
heap.c (r1.265 -> r1.266)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/catalog/heap.c.diff?r1=1.265&r2=1.266)
index.c (r1.232 -> r1.233)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/catalog/index.c.diff?r1=1.232&r2=1.233)
pgsql-server/src/backend/commands:
cluster.c (r1.124 -> r1.125)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/commands/cluster.c.diff?r1=1.124&r2=1.125)
vacuum.c (r1.278 -> r1.279)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/commands/vacuum.c.diff?r1=1.278&r2=1.279)
vacuumlazy.c (r1.40 -> r1.41)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/commands/vacuumlazy.c.diff?r1=1.40&r2=1.41)
pgsql-server/src/backend/storage/buffer:
bufmgr.c (r1.167 -> r1.168)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/storage/buffer/bufmgr.c.diff?r1=1.167&r2=1.168)
pgsql-server/src/include/storage:
bufmgr.h (r1.81 -> r1.82)
(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/storage/bufmgr.h.diff?r1=1.81&r2=1.82)