pgsql: Code review for escape-strings patch.

Log Message:
-----------
Code review for escape-strings patch.  Sync psql and plpgsql lexers
with main, avoid using a SQL-defined SQLSTATE for what is most definitely
not a SQL-compatible error condition, fix documentation omissions,
adhere to message style guidelines, don't use two GUC_REPORT variables
when one is sufficient.  Nothing done about pg_dump issues.

Modified Files:
--------------
    pgsql/doc/src/sgml:
        errcodes.sgml (r1.12 -> r1.13)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/errcodes.sgml.diff?r1=1.12&r2=1.13)
        libpq.sgml (r1.186 -> r1.187)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/libpq.sgml.diff?r1=1.186&r2=1.187)
        protocol.sgml (r1.59 -> r1.60)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/protocol.sgml.diff?r1=1.59&r2=1.60)
        runtime.sgml (r1.331 -> r1.332)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/runtime.sgml.diff?r1=1.331&r2=1.332)
        syntax.sgml (r1.101 -> r1.102)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/doc/src/sgml/syntax.sgml.diff?r1=1.101&r2=1.102)
    pgsql/src/backend/parser:
        scan.l (r1.126 -> r1.127)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/parser/scan.l.diff?r1=1.126&r2=1.127)
    pgsql/src/backend/utils/misc:
        guc.c (r1.269 -> r1.270)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/backend/utils/misc/guc.c.diff?r1=1.269&r2=1.270)
    pgsql/src/bin/psql:
        psqlscan.l (r1.14 -> r1.15)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/bin/psql/psqlscan.l.diff?r1=1.14&r2=1.15)
    pgsql/src/include/utils:
        errcodes.h (r1.17 -> r1.18)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/include/utils/errcodes.h.diff?r1=1.17&r2=1.18)
    pgsql/src/pl/plpgsql/src:
        scan.l (r1.41 -> r1.42)
        (http://developer.postgresql.org/cvsweb.cgi/pgsql/src/pl/plpgsql/src/scan.l.diff?r1=1.41&r2=1.42)