[8.x] Fix display of validation errors occurred when asserting status

[jessarcher]

As discovered by @paulredmond, the display of unexpected validation errors introduced in #38046 doesn't actually work in practice because a ValidationException is not reported by the exception handler. Paul's diagnosis can be found here. Thankfully this doesn't actually break any existing behaviour.

This PR fixes the issue by also showing errors from a redirect response with session errors (i.e. a non-JSON validation response) and errors in JSON response content, instead of only looking at reported exceptions.

I have learned my lesson and tested this in a new laravel/laravel project.

Errors in redirect response session:

Errors in response JSON content:

The behaviour is slightly different than before because assertStatus() no longer only displays errors for unexpected 500 and 422 responses, and instead just displays any reported exceptions, session errors, or JSON errors when the status is not as asserted. This also makes assertStatus() behave the same as assertSuccessful().

I also took the opportunity to standardise the messaging of "Expected response status code" which in some instances was just "Expected status code".