Error message testing fix

The testing of error messages have been implemented wrongly a few times.
This is an attempt to fix it.

For example, some of these test should have failed with the new code.
The reason they are not failling with the new string is the fact they
were not being tested beforehand.
You should be able to use `assert_includes` with the string:

```ruby
"Missing partial /_missing with {:locale=>[:en], :formats=>[:json], :variants=>[], :handlers=>[:erb, :builder, :raw, :ruby]}."
```