Second part of #3, error-check to cover normal application launches.

In #3 we extended the JUnit launcher to check that there are no build problems
before a launch. Unfortunately there is no good way to share code between the
two launchers, so the same fix is now extended to the normal Equinox launcher.
Merge pull request #5 from dragos/issue/launch-errors-3

Second part of #3, error-check to cover normal application launches.