Fix runtime path to sandbox.rkt
Don't exclude racket/sandbox. We no longer use it.
Ensure error messages provide full pathnames.

So that compilation-mode use them.

This is a loose end related to commit a691936.
Ignore all srclocs for exn:fail:syntax?

This is especially helpful for Typed Racket, as it may provide multiple
srclocs, but they are already included in exn-message so we don't
need/want them printed out again at the bottom.