[data][RemoteException] Fix __str__

The way the string returned by __str__ is created fails with
Python 3.

beat/backend/python/data.py

@@ -65,10 +65,10 @@ class RemoteException(Exception):
             self.user_error = message
 
     def __str__(self):
-        if self.system_error != '':
-            return '(sys) ' + self.system_error
+        if self.system_error:
+            return '(sys) {}'.format(self.system_error)
         else:
-            return '(usr) ' + self.user_error
+            return '(usr) {}'.format(self.user_error)
 
 
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