avocado.core.exceptions: Add OptionValidationError

So we can let the test runner to notify the user of
incorrect options passed in.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
avocado.job: Give a better message to user when empty test URL is provided

Give the user a nice message rather than a crash report.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>
selftests: Add more functional tests

Add tests to verify if passing the full test path
works, rather than the test ID, and if an empty
test ID/alias triggers the correct error code.

Signed-off-by: Lucas Meneghel Rodrigues <lmr@redhat.com>