Gracefully handle unknown JDK classpath containers.

If the JDK required by the project does not exist, `getClasspathContainer` returns null.

Fixed #1001249.
No need to block the whole workspace to add a marker.

Resources do the necessary scheduling already inside `createMarker`.
Formatting.
Issue errors if the class path contains cross-compiled libraries with a different major version of the Scala compiler.

Identify jars with `name_2.x.y-??.jar` and compare the plugin version with the one in the name, issue errors for each library that is not compatible.

Fixed #1001441.
Reviewer's comments.
Merge pull request #286 from dragos/issue/classpath-binary-validation

Validate cross-compiled binaries on the classpath