restore accidentally removed EngineError

in cleanup PR  #3527, I removed one too many exception classes.

I acked IPython to confirm that I didn't miss any more.

closes  #4100

should be backported to 1.1.

IPython/parallel/error.py

@@ -43,6 +43,9 b' class IPythonError(Exception):'
 class KernelError(IPythonError):
     pass
 
+class EngineError(KernelError):
+    pass
+
 class NoEnginesRegistered(KernelError):
     pass