fix bug in online check syntax's save file notification

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
move the ╬ char and friends into a separate file

do not merge to 5.3.2