Add failure summary report to spec_reporter.

@os97673 

Scrolling through hundreds -- or in our case thousands -- of test results to locate the failure message for one or a handful of tests is rather painful.  This adds an error summary report to the end of the spec reporter, but before the final test summary is printed.  It omits skipped tests.

You can view the resulting output of this PR by running `bundle exec rake gallery`, or:

```
  1) Failure: MinitestReportersTest::BadTest#test_a [/Users/timmertins/github/minitest-reporters/test/gallery/bad_test.rb:6]:
        Expected: 1
          Actual: 2

  2) Failure: MinitestReportersTest::BadTest#test_b [/Users/timmertins/github/minitest-reporters/test/gallery/bad_test.rb:14]:
        --- expected
        +++ actual
        @@ -1,2 +1,2 @@
         "ab
        -c"
        +d"

  3) Error: MinitestReportersTest::BadTest#test_boom:
        RuntimeError: A random exception
            /Users/timmertins/github/minitest-reporters/test/gallery/bad_test.rb:18:in `test_boom'

Finished in 0.01229s
6 tests, 5 assertions, 2 failures, 1 errors, 1 skips
```