Ensure that a base language is loaded before custom theme

Main goal of this patch is to improve error handling when a custom
theme does not contain a valid theme version. We were getting a
message saying that the theme_ver_err resource did not exist as
well as an error saying that the package had failed to load. Now
we properly get a single error message about the theme version.

Change-Id: I361ef2d139220bbeaf2e856be919dc3a50ae4299