Issue/launch config does not find main in jar 1001878

### Dumbed down `MainClassVerifier` to only check class vs module mismatch …

Experience taught us that `MainClassVerifier` is trying to be too smart, and
often prevents user from launching perfectly fine Scala launch configurations.
The last manifestation was that it would prevent users from launching a main if
defined in a JAR.

The matter was discussed with the team and the agreed descision is to reduce
the scope of the `MainClassVerifier` to only prevent users from:

* launching a Scala application if the project contains build errors, and
* provide feedback when trying to launch a Scala `class`, instead of a `object`

Consequently, we no longer prevent users from launching a Scala application
that is incorrectly configured (i.e., the classfile doesn't exist). The
consequence is that a `NoClassFoundException` will be reported to the user in
this case, which actually matches the behavior in the Eclipse Java support.

backport to release/scala-ide-3.0.x

Fixes #1001878