Fix root cause of #85 in CM-MJ:

  > Fix `TypeError: Cannot read property 'line' of undefined` exception. Triggered on any change when line 0 contains a CM bookmark.
  > Catch all errors in our CM event handlers so (I hope) we can't ever break other code handling CM events, such as Firepad as happened in #85.
and another dumb bug from same commit:
  > Fix #19 correctly: clear only our markers, not only foreign ones!