Fifth version :)

- On failure in a loaded test, the _test function returns, so this behaviour is preserved.
- During _test function, count of both loaded and non-loaded files is kept.
- If a file fails to be loaded, the tests continue, but a non-zero return code is returned. Moreover, the results of loaded and non-loaded files are printed after each invocation.

This is a sample output:


$ python3 -m doctest -v spam c.py beans
Cannot read 'spam': [Errno 2] No such file or directory: 'spam'
Trying:
    2 * 2
Expecting:
    4
ok
1 items passed all tests:
   1 tests in c
1 tests in 1 items.
1 passed and 0 failed.
Test passed.
Cannot read 'beans': [Errno 2] No such file or directory: 'beans'
Test files read successfully: 1
Unreadable files: 2


Does this look better?