[FLINK-10695] Cannot load user class errors set underlying cause

[A412]

What is the purpose of the change

• This pull request improves the traceability of "Cannot load user class" exceptions.

Brief change log

• Set the underlying ClassNotFoundExceptions as the cause of the resulting StreamTaskExceptions.

Verifying this change

This change is a trivial rework / code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

• Does this pull request introduce a new feature? (yes / no)
• If yes, how is the feature documented? (not applicable / docs / JavaDocs / not documented)

[tillrohrmann]

Thanks for the fix @A412. LGTM. Merging.