pgsql-server: Code review for NOWAIT patch: downgrade NOWAIT from fully

Log Message:
-----------
Code review for NOWAIT patch: downgrade NOWAIT from fully reserved keyword
to unreserved keyword, use ereport not elog, assign a separate error code
for 'could not obtain lock' so that applications will be able to detect
that case cleanly.

Modified Files:
--------------
    pgsql-server/doc/src/sgml:
        errcodes.sgml (r1.8 -> r1.9)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/doc/src/sgml/errcodes.sgml.diff?r1=1.8&r2=1.9)
    pgsql-server/doc/src/sgml/ref:
        lock.sgml (r1.42 -> r1.43)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/doc/src/sgml/ref/lock.sgml.diff?r1=1.42&r2=1.43)
    pgsql-server/src/backend/access/heap:
        heapam.c (r1.176 -> r1.177)

(http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/access/heap/heapam.c.diff?r1=1.176&r2=1.177)
    pgsql-server/src/backend/parser:
        gram.y (r2.477 -> r2.478)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/backend/parser/gram.y.diff?r1=2.477&r2=2.478)
    pgsql-server/src/include/utils:
        errcodes.h (r1.15 -> r1.16)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/include/utils/errcodes.h.diff?r1=1.15&r2=1.16)
    pgsql-server/src/pl/plpgsql/src:
        plerrcodes.h (r1.4 -> r1.5)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql-server/src/pl/plpgsql/src/plerrcodes.h.diff?r1=1.4&r2=1.5)