Improve error message for missing extension.

Improve error message for missing extension.

If we get ENOENT while trying to read an extension control file,
report that as a missing extension (with a HINT to install it)
rather than as a filesystem access problem.  The message wording
was extensively bikeshedded in hopes of pointing people to the
idea that they need to do a software installation before they
can install the extension into the current database.

Nathan Bossart, with review/wording suggestions from Daniel
Gustafsson, Chapman Flack, and myself

Discussion: https://postgr.es/m/3950D56A-4E47-48E7-BF9B-F5F22E268BE7@amazon.com