add better error for passing config with no steps

Summary: in response to https://github.com/facebook/openr/issues/23

Reviewed By: snarkmaster

Differential Revision: D7342371

fbshipit-source-id: 15737f79900722ce016494f994932aa26e5acad5

build/fbcode_builder/fbcode_builder.py

@@ -138,6 +138,9 @@ class FBCodeBuilder(object):
         return res
 
     def build(self, steps):
+        if not steps:
+            raise RuntimeError('Please ensure that the config you are passing '
+                               'contains steps')
         return [self.setup(), self.diagnostics()] + steps
 
     def setup(self):