Better exception message for invalid JSON in hint distro file

[fenhl]

A user of my fork DM'd me today because they had run into an issue where the randomizer showed this error message:

json.decoder.JSONDecodeError: Expecting value: line 25 column 5 (char 1556)

The problem was a syntax error in one of their custom hint distribution files. The error message does not mention this; you need to read the traceback to find the cause of the issue. This PR changes the error message to the following:

ValueError: Could not parse hint distribution file 'syntax_error.json'. Make sure the file is valid JSON or reach out to Support on Discord for help. Details: Expecting value: line 25 column 5 (char 1556)

This can be tested by placing a hint distribution that's not valid JSON (e.g. an empty file) into data/Hints.