Fix output of Minitest::UnexpectedErrors.

Need to get the underlying exception otherwise the backtrace filtering
and printing occurs twice as it's also done by
UnexpectedError.message.

This may well be the wrong approach so feel free to ask me to fix it another way.

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'
```