Before this commit, the files that are loaded during expansion
were discarded when an error during expansion occurs. This commit
saves them: unless the program is something like
(begin-for-syntax (kill-thread (current-thread)))
the handling that deals with that kind of situation (as opposed
to just an exception being raised) doesn't try to save them
Do not merge to 5.3.2: this bug isn't serious and the new code
is not as well tested as the old