Improve error messages for invalid arguments

[senofsky]

Currently, _validate_args() will log an error stating that a directory is not writeable if it does not exist.

While this is technically true, we should notify the users why the directory isn't writeable; notifying the user that the directory does not exist or the permissions are set to read-only is more helpful.

We should always strive to provide a positive user experience.

[senofsky]

Update: A couple of things have changed since this issue was created:

• _validate_args no longer exists
• command_line_interface.py interfaces with the user

However, we still need better exception handling logic in command_line_interface.py. For example:

face-mask-detector/face_mask_detector/lib.py

Line 196 in cae54a3

image = cv2.imread(image_path)

doesn't check if the image path actually exists - this will result in a FileNotFoundError exception.

This is just one of many possible exceptions that need to be handled. The user should be informed that the image path they gave either does not exist, has permissions that prevents face-mask-detector from opening it, or has some other issue.

We need to catch every possible exception that can be raised and print a helpful error message to the user.