Fix Python 3 attribute error in test.

Author: theengineear

plotly/exceptions.py

@@ -54,7 +54,10 @@ def __init__(self, requests_exception):
         elif content_type == 'text/plain':
             self.message = requests_exception.response.content
         else:
-            self.message = requests_exception.message
+            try:
+                self.message = requests_exception.message
+            except AttributeError:
+                self.message = 'unknown error'
 
     def __str__(self):
         return self.message