Resolve TypeError when raising GraphQL errors

`results.errors[0]` is a dict and raising it throws a TypeError:
`TypeError: exceptions must be old-style classes or derived from BaseException, not dict`

This fixes this error.
Granted its not the best solution and we should probably subclass Exception to our own GQLError or something in the future.
But at least it makes the current code work and provide error feedback and not a TypeError

Author: ekampf

gql/client.py

@@ -49,7 +49,7 @@ def execute(self, document, *args, **kwargs):
 
         result = self._get_result(document, *args, **kwargs)
         if result.errors:
-            raise result.errors[0]
+            raise Exception(str(result.errors[0]))
 
         return result.data