Do a fresh compile for preview after error (#9061)

Do a fresh compile for preview after error (#9061)

As Enrico said, the user might have installed a package that was
missing (in which case the .tex file would not have changed).

Another reason is that changing some document settings did not
automatically lead to a fresh compile after an error (#9061).

Our old mechanism for detemining whether there was an error was to
check if the dependent file existed in the temporary directory. If
it did not exist, that meant it was removed, presumably because
there was an error during compilation. That mechanism cannot be used
anymore because we keep the files around even after error because of
the "Show Output Anyway" button (09700d5b). This commit implements a
more straightforward way of checking whether there was an error in
the previous preview by simply storing the success of last compile
in a buffer variable.