Homogeneize error messages.

src/dynamic_graph/entity.py

@@ -155,7 +155,7 @@ class Entity (object) :
             try:
                 object.__getattr__(self, name)
             except AttributeError:
-                raise AttributeError('Entity named "%s" has no attribute %s\n'%
+                raise AttributeError("'%s' entity has no attribute %s\n"%
                                      (self.name, name)+
                                      '  entity attributes are usually either\n'+
                                      '    - commands,\n'+

src/entity-py.cc

@@ -207,9 +207,10 @@ namespace dynamicgraph {
 	  entity->getNewStyleCommandMap();
 
 	if (commandMap.count(std::string(commandName)) != 1) {
-	  std::string msg = "command " + std::string(commandName) +
-	    " is not referenced in Entity " + entity->getName();
-	  PyErr_SetString(dgpyError, msg.c_str());
+	  std::ostringstream oss;
+	  oss << "'" << entity->getName() << "' entity has no command '"
+	      << commandName << "'.";
+	  PyErr_SetString(PyExc_AttributeError, oss.str().c_str());
 	  return NULL;
 	}
 	Command* command = commandMap[std::string(commandName)];