fix(scale): return error message in proper format

Author: Keerthan Mala

rootfs/api/models/app.py

@@ -317,7 +317,7 @@ def scale(self, user, structure):  # noqa
             for target, count in structure.copy().items():
                 structure[target] = int(count)
             validate_app_structure(structure)
-        except (TypeError, ValueError) as e:
+        except (TypeError, ValidationError) as e:
             raise DeisException('Invalid scaling format: {}'.format(e))
 
         # test for available process types