add attributes to get error name and value

Author: rohitsanj

nbclient/exceptions.py

@@ -40,9 +40,11 @@ class CellExecutionError(Exception):
     failures gracefully.
     """
 
-    def __init__(self, traceback):
+    def __init__(self, traceback, ename, evalue):
         super(CellExecutionError, self).__init__(traceback)
         self.traceback = traceback
+        self.ename = ename
+        self.evalue = evalue
 
     def __str__(self):
         s = self.__unicode__()
@@ -65,8 +67,10 @@ def from_cell_and_msg(cls, cell, msg):
                 traceback=tb,
                 ename=msg.get('ename', '<Error>'),
                 evalue=msg.get('evalue', ''),
-            )
-        )
+            ),
+            ename=msg.get('ename', '<Error>'),
+            evalue=msg.get('evalue', '')
+    )
 
 
 exec_err_msg = u"""\