test_slapgrid: return 404 instead of 500 if url is not handled

slapos/tests/test_slapgrid.py

@@ -494,7 +494,7 @@ class ComputerForTest(object):
         return {'status_code': self.status_code}
 
     else:
-      return {'status_code': 500}
+      return {'status_code': 404, 'reason': 'test_slapgrid does not handle'}
 
   def getTestSoftwareClass(self):
     return SoftwareForTest