Browsing Bulgarian translation

Developers may run into this if they're naively
trying to run from the build-tree without having
specifically configured the build to allow that or
having passed a "--gui=..." option to the program,
so we need to actually catch this error and output
an informative message.

This is, however, not an error that end users should face;
if they do, then someone upstream from them messed up--
either `make install' worked only half-way, or a packager
left something critical out of the package. In either case,
we want to fail in a way that indicates to the user
that something is really wrong and they should report
a bug.