Provide better message when CREATE EXTENSION can't find a target schema.
Provide better message when CREATE EXTENSION can't find a target schema.

The new message (and SQLSTATE) matches the corresponding error cases in
namespace.c.

This was thought to be a "can't happen" case when extension.c was written,
so we didn't think hard about how to report it.  But it definitely can
happen in 9.2 and later, since we no longer require search_path to contain
any valid schema names.  It's probably also possible in 9.1 if search_path
came from a noninteractive source.  So, back-patch to all releases
containing this code.

Per report from Sean Chittenden, though this isn't exactly his patch.

jmcatamney/gpdb,oberstet/postgres-xl,greenplum-db/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,ovr/postgres-xl,zeroae/postgres-xl,greenplum-db/gpdb,lisakowen/gpdb,pavanvd/postgres-xl,adam8157/gpdb,techdragon/Postgres-XL,yazun/postgres-xl,ashwinstar/gpdb,oberstet/postgres-xl,zeroae/postgres-xl,50wu/gpdb,lisakowen/gpdb,zeroae/postgres-xl,lisakowen/gpdb,Postgres-XL/Postgres-XL,jmcatamney/gpdb,jmcatamney/gpdb,techdragon/Postgres-XL,greenplum-db/gpdb,pavanvd/postgres-xl,lisakowen/gpdb,adam8157/gpdb,ovr/postgres-xl,zeroae/postgres-xl,Postgres-XL/Postgres-XL,xinzweb/gpdb,Postgres-XL/Postgres-XL,50wu/gpdb,yazun/postgres-xl,50wu/gpdb,pavanvd/postgres-xl,xinzweb/gpdb,ashwinstar/gpdb,50wu/gpdb,xinzweb/gpdb,adam8157/gpdb,50wu/gpdb,adam8157/gpdb,ovr/postgres-xl,adam8157/gpdb,Postgres-XL/Postgres-XL,greenplum-db/gpdb,greenplum-db/gpdb,adam8157/gpdb,techdragon/Postgres-XL,ovr/postgres-xl,techdragon/Postgres-XL,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,oberstet/postgres-xl,greenplum-db/gpdb,lisakowen/gpdb,oberstet/postgres-xl,xinzweb/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,jmcatamney/gpdb,ashwinstar/gpdb,ovr/postgres-xl,greenplum-db/gpdb,ashwinstar/gpdb,yazun/postgres-xl,oberstet/postgres-xl,pavanvd/postgres-xl,ashwinstar/gpdb,adam8157/gpdb,ashwinstar/gpdb,techdragon/Postgres-XL,jmcatamney/gpdb,Postgres-XL/Postgres-XL,pavanvd/postgres-xl,xinzweb/gpdb,yazun/postgres-xl,ashwinstar/gpdb,xinzweb/gpdb,jmcatamney/gpdb,yazun/postgres-xl,50wu/gpdb,lisakowen/gpdb,lisakowen/gpdb,jmcatamney/gpdb,zeroae/postgres-xl,50wu/gpdb