If a theme was not a file or a directory you could get a crash or a hang
(depending on platform) if the theme references a directory. This patch
also prevents attempts to load from other non-file sources.
Fixes: #5596
Rather than storing a list of errors we now store a list of
"diagnostics." Each diagnostic has a richer set of structured
information, including a message, a key, the location where it occurred.
This lets us show more detailed messages, more human friendly messages, and
also let's us filter by key or location. We don't take advantage of
all of this capability in this initial commit, but we do use every field
for something.
