[SPARK-15288][MESOS] Mesos dispatcher should handle gracefully when any thread gets UncaughtException

## What changes were proposed in this pull request?

Adding the default UncaughtExceptionHandler to the MesosClusterDispatcher.
## How was this patch tested?

I verified it manually, when any of the dispatcher thread gets uncaught exceptions then the default UncaughtExceptionHandler will handle those exceptions.

Author: Devaraj K <devaraj@apache.org>

Closes #13072 from devaraj-kavali/SPARK-15288.