DEV: Disable better_errors when developing with more than one worker (PR #8201)

From the better_errors README:

Better Errors works by leaving a lot of context in server process memory. If you’re using a web server that runs multiple “workers” it’s likely that a second request (as happens when you click on a stack frame) will hit a different worker. That worker won’t have the necessary context in memory, and you’ll see a Session Expired message.

GitHub

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Makes sense! I’ve often wondered why I don’t get error data from time to time.