pgsql: Provide better message when CREATE EXTENSION can't find a target

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.

Branch
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REL9_1_STABLE

Details
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http://git.postgresql.org/pg/commitdiff/f94979f618abffb63cad2266f39db8beeeb18afc

Modified Files
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src/backend/commands/extension.c | 10 +++++++---
1 file changed, 7 insertions(+), 3 deletions(-)