`(ulimit -n 30; make test-tool)` fails with unexplicit message:
"undefined method `write' for nil:NilClass" due to lack of stdout.

This change makes it explicit.  [Bug #5577]

`Exception#full_message` is only defined on ruby 2.5.0 and above.

https://github.com/ruby/fileutils/commit/a8968f41ed

`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>.
```

https://github.com/ruby/fileutils/commit/2f38ba6e82

http://ci.rvm.jp/results/trunk-mjit@silicon-docker/2380788
```
test_all             #<Thread:0x000055b6c8e9fca8@/tmp/ruby/v2/src/trunk-mjit/tool/lib/test/unit/parallel.rb:42 run> terminated with exception (report_on_exception is true):
<internal:pack>:134:in `pack': no implicit conversion of false into String (TypeError)
	from /tmp/ruby/v2/src/trunk-mjit/tool/lib/test/unit/parallel.rb:160:in `_report'
	from /tmp/ruby/v2/src/trunk-mjit/tool/lib/test/unit/parallel.rb:45:in `block in _run_suite'
```

This reverts commit 43015275b9a7f2833c93ad11ea96ae4cb3b7acd7.

`assert_raise_message` in test-unit is different from
`assert_raise_with_message`.  It checks the exception message
only, but not the exception class,