In D18797, I introduced a `catch (ParseError ...)` to deal with a PHP7 special case. However, this class won't exist in earlier versions of PHP like PHP5.
@lpriestley reports hitting an error with this under PHP5 ("unknown class/interface ParseError") from the linter while running `arc diff`, which is expected since I didn't add it to the secret list of special PHP7 magic.
Add it to the PHP7 magic.
(It's fine that we catch something which may not exist in PHP5, since `Throwable` does not exist in PHP5 and the `catch (Exception ...)` above will catch everything which can be caught in PHP5.)