[bug] mdrip test panics if there are no md files.

[jayunit100]

problem statement

mdrip test panics if run with a folder that has no .md files in it, that gives users impression that mdrip itself is broken.

solution

• mdrip test should pass if there are no .md files

We can implement this, by having the Loader visitFiles/ function return an error in a way which prevents a panic.