Improved error message for CREATE EXTENSION patch...

I ran in to the following situation:

> SET search_path = ENOENT, also_does_not_exist;
> CREATE EXTENSION pg_repack;
> ERROR:  XX000: there is no default creation target
> LOCATION:  CreateExtension, extension.c:1395

Which left me checking out the source code to figure out exactly what the problem was. Attached is an improved error
message:

> "ERROR:  XX000: no schemas in search_path are available for CREATE EXTENSION"

-sc



--
Sean Chittenden
sean@chittenden.org