[FLINK-10695] "Cannot load user class" errors should set the underlyi…

…ng ClassNotFoundException as their cause

This closes apache#6945.

flink-streaming-java/src/main/java/org/apache/flink/streaming/api/graph/StreamConfig.java

@@ -233,7 +233,7 @@ public <T extends StreamOperator<?>> T getStreamOperator(ClassLoader cl) {
  "\nClass was actually found in classloader - deserialization issue." :
  "\nClass not resolvable through given classloader.");
 
- throw new StreamTaskException(exceptionMessage);
+ throw new StreamTaskException(exceptionMessage, e);
  }
  catch (Exception e) {
  throw new StreamTaskException("Cannot instantiate user function.", e);