Fix output of Minitest::UnexpectedErrors.

No need to filter the backtrace, print the class or message as these are already done by `UnexpectedError` (which wraps all errors as-of Minitest 5).

Without this change:
```
Error:
TestSomething#test_something:
Minitest::UnexpectedError: RuntimeError: 
    /Users/mike/something/something_test.rb:24:in `test_something'
    /Users/mike/something/something_test.rb:24:in `test_something'
```

With this change:
```
Error:
TestSomething#test_something:
RuntimeError: 
    /Users/mike/something/something_test.rb:24:in `test_something'
```