Check return value of doctest.testmod()

Adapted from 41afa16500

kafka-merge-pr.py

@@ -440,6 +440,8 @@ def main():
 
 if __name__ == "__main__":
     import doctest
-    doctest.testmod()
+    (failure_count, test_count) = doctest.testmod()
+    if (failure_count):
+        exit(-1)
 
     main()