[algorithm] Improve error message if algorithm is not valid on export

beat/backend/python/algorithm.py

@@ -912,7 +912,7 @@ class Algorithm(object):
             raise RuntimeError("algorithm has no name")
 
         if not self.valid:
-            raise RuntimeError("algorithm is not valid")
+            raise RuntimeError("algorithm is not valid:\n%s" % "\n".join(self.errors))
 
         if prefix == self.prefix:
             raise RuntimeError("Cannot export algorithm to the same prefix ("