Add LINE: hint when schemaname.typename is a non-existent schema

The attached patch adds a LINE: ... hint when schemaname.typename
results in a schema which does not exist. I came across this when a
missing comma in a SELECT list resulted in an error without a location
in a query a few thousand lines long.

Before:

(postgres@[local]:5432 14:41:25) [postgres]> select test.id 'all' as
example from test;
ERROR:  3F000: schema "test" does not exist
LOCATION:  get_namespace_oid, namespace.c:2826

After:

(postgres@[local]:5433 14:42:32) [postgres]> select test.id 'all' as
example from test;
ERROR:  3F000: schema "test" does not exist
LINE 1: select test.id 'all' as example from test;
               ^
LOCATION:  LookupTypeName, parse_type.c:171

-Ryan Kelly