Improve error on malformed known_package_places

[Freso]

The current traceback on a bad known_package_places file doesn’t make it immediately obvious that it’s a local file’s JSON that has been garbled rather than a response from AURweb or elsewhere that’s gone awry.

This commit adds a single message to the JSON decode error to note which file the error is happening in and the user can then either delete the file or try and fix it, depending on the state of the file in question.