Improve error message related to usage variables starting with '_'

smnp/runtime/evaluators/assignment.py

@@ -9,7 +9,7 @@ class AssignmentEvaluator(Evaluator):
     def evaluator(cls, node, environment):
         target = node.target.value
         if target.startswith("_"):
-            raise RuntimeException("Usage of variables with names starting with '_' is not allowed", node.target.pos)
+            raise RuntimeException("Declaration and assignation variables with names starting with '_' is not allowed", node.target.pos)
 
         value = expressionEvaluator(doAssert=True)(node.value, environment).value #TODO check if it isn't necessary to verify 'result' attr of EvaluatioNResult
         scopeOfExistingVariable = environment.findVariableScope(target)