FIX: Provide informative error for interfaces that fail to return runtime object

Author: effigies

nipype/interfaces/base/core.py

@@ -540,6 +540,9 @@ def run(self, cwd=None, ignore_exception=None, **inputs):
             if not ignore_exception:
                 raise
         finally:
+            if runtime is None:
+                raise RuntimeError("{} interface failed to return runtime "
+                                   "object".format(interface.__class__.__name__))
             # This needs to be done always
             runtime.endTime = dt.isoformat(dt.utcnow())
             timediff = parseutc(runtime.endTime) - parseutc(runtime.startTime)