improve CREATE EXTENSION error message

Hi hackers,

Currently, if you attempt to use CREATE EXTENSION for an extension
that is not installed, you'll see something like the following:

        postgres=# CREATE EXTENSION does_not_exist;
        ERROR:  could not open extension control file "/usr/local/pgsql/share/extension/does_not_exist.control": No
suchfile or directory
 

I suspect this ERROR message is confusing for novice users, so perhaps
we should add a HINT.  With the attached patch, you'd see the
following:

        postgres=# CREATE EXTENSION does_not_exist;
        ERROR:  could not open extension control file "/usr/local/pgsql/share/extension/does_not_exist.control": No
suchfile or directory
 
        HINT:  This typically indicates that the specified extension is not installed on the system.

Thoughts?

Nathan