`exception_details` is not defined anywhere.
This commit fixes the following test crash in ruby 2.4
```
Error: test_assert_output_lines(TestFileUtils): NoMethodError: undefined method `exception_details' for #<TestFileUtils:0x00005556ef699178>
```
And replaces it with an actual test failure:
```
[Test::Unit::CoreAssertions::MiniTest::Assertion] exception expected, not #<NoMethodError: undefined method `full_message' for #<RuntimeError: ok>
Did you mean? message>.
```
2f38ba6e82
This reverts commit 43015275b9.
`assert_raise_message` in test-unit is different from
`assert_raise_with_message`. It checks the exception message
only, but not the exception class,
