Enhance old release version handling in install.sh

[sygibson]

Currently, the install.sh script exits with an unfriendly error if an older version is specified than is available in the Catalog, like:

EXIT MESSAGE: FATAL: Internal error, get() expects files to get

The Installer should handle this a little more gracefully if possible. Currently at line 731 - we grep through the catalog to find drp-$DRP_VERSION - which may return an empty string, this is passed to the get() call without checking.

Error messaging should be enhanced to catch this.