Catch unexpected errors reading config file

[unflxw]

When reading the __appsignal__.py configuration file from the demo or diagnose command, handle unexpected errors in the file (e.g. syntax errors) instead of only handling the expected error of the appsignal variable being missing.

Unify the handling of errors when reading a config file. Provide an ExitError that can be raised by command helpers to trigger the command to exit.

Since loading the configuration file is only done by the diagnose and demo commands, move that functionality out of the Client file and into a helper for the CLI commands.

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