FS#13746: open_plugin_run() doesn't handle "not found" properly
If the specified key wasn't found, we would fall back to trying to launch
a plugin with the LANG_* key. This will fail, resulting in an error
that spells out the (inappropriate) LANG_* text.
Instead, fail out with a more appropriate error.
Change-Id: I046878f87a2ef2bc994d0b7d037359695a18d392