When loading tests, check if the test directory exists.

[elliotboney]

This prevents an uncaught exception if you have a test type in one site, and not another.

As an example, in Site #A, a user has an "acceptance" suite that they don't have in Site #B. When Codeception loadTests() is called, it iterates through the 'tests' array in config, in each site directory, and crashes if the directory doesn't exist.

Let me know if this needs more clarification.

[elliotboney]

This also would close #35

[Patabugen]

This is a great PR which I'd love to merge - but it's got unrelated fixes/changes involved.

Could you: git checkout -b check_directory_exists jayhealey/master && git push origin check_directory_exists (note tested but should work!)

so there's a branch containing only that fix, I can then merge the pull request.