Quick fix exceptions

The first commit wraps the return value of `WordQuickFixProcessor.getCorrections` in a Option, as it can return 'null'.
The second one tweaks the call to SafeRunner to avoid to have a duplicate stack trace in case of problem.

The original goal was to fix tickets [1002317](https://scala-ide-portfolio.assembla.com/spaces/scala-ide/tickets/1002317) and [1002281](https://scala-ide-portfolio.assembla.com/spaces/scala-ide/tickets/1002281), but the real problem lies between Scala compiler and Scala refactoring (scala-ide/scala-refactoring#67).