Fail Gracefully when Deepbooru Can't Guess Tags

[testIgnor]

Currently, if tags doesn't become populated, then the following code path is taken:

The following if statement prints a warning message with no other logic:

szurubooru-toolkit/src/szurubooru_toolkit/deepbooru.py

Line 102 in 2ec633d

if not tags:

Because the else block is ignored, rating is not assigned a value before being referenced, causing the script to crash here:

szurubooru-toolkit/src/szurubooru_toolkit/deepbooru.py

Line 119 in 2ec633d

if rating is None:

This can be handled better by

1. Providing a more informative error message. Perhaps something along the lines of f'Deepbooru could not guess tags for image! Maximum inference was {np.amax(results)}, while threshold is {threshold}!' Would also be helpful to print which image caused the error.
2. Returning early within the if not tags block.