Add error_trace to string representation of an Error

If the `error_trace` payload is available, add it to the string
representation of the Error class.

src/crate/client/exceptions.py

@@ -30,6 +30,10 @@ def __init__(self, msg=None, error_trace=None):
         super(Error, self).__init__(msg)
         self.error_trace = error_trace
 
+    def __str__(self):
+        if self.error_trace is None:
+            return super().__str__()
+        return super().__str__() + str(self.error_trace)
 
 class Warning(Exception):
     pass