Fix compatibility with Python 2.7 and improve warning message.

Now the rule identifier is printed as part of the warning message, the string identifier is not descriptive enough in many cases.

yara-python.c

@@ -625,8 +625,7 @@ int yara_callback(
 
   // If the rule doesn't match and the user has not specified that they want to
   // see non-matches then nothing to do here.
-  if (message == CALLBACK_MSG_RULE_NOT_MATCHING &&
-      callback != NULL &&
+  if (message == CALLBACK_MSG_RULE_NOT_MATCHING && callback != NULL &&
       (which & CALLBACK_NON_MATCHES) != CALLBACK_NON_MATCHES)
     return CALLBACK_CONTINUE;
 
@@ -725,14 +724,18 @@ int yara_callback(
 
     if (warning_callback == NULL)
     {
-      // If the user doesn't provide a callback we will print a warning.
-      // NOTE: PyErr_WarnFormat is new in python 3.2. I can go with something
-      // more portable if desired.
-      result = PyErr_WarnFormat(
-          PyExc_RuntimeWarning,
-          1,
-          "maximum matches for string %s. Results may be invalid.",
-          ((YR_STRING*) message_data)->identifier);
+      char message[200];
+
+      string = (YR_STRING*) message_data;
+
+      snprintf(
+          message,
+          sizeof(message),
+          "too many matches for string %s in rule \"%s\"",
+          string->identifier,
+          context->rules->rules_table[string->rule_idx].identifier);
+
+      result = PyErr_WarnEx(PyExc_RuntimeWarning, message, 1);
 
       if (result == -1)
         result = CALLBACK_ERROR;