Print module exception by default

At the moment mavproxy does not print module exceptions by default. This is especially annoying if you try a module for the first time and something is not working or there is a bug in an old module. I suggest that mavproxy displays a minimal error message if an exception happens and enables more detailed debugging with the `moddebug` option.

E.g. https://github.com/ArduPilot/MAVProxy/blob/master/MAVProxy/modules/mavproxy_link.py#L908 would need a change.

Start with MAVProxy/modules/mavproxy_link.py around line 908 and trace how module exceptions are handled, including the existing moddebug option. Confirm the intended default minimal error output and the more detailed output when moddebug is enabled. Done means module exceptions are visible by default without requiring debugging to be enabled.

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