Add Exception message

[vetinari]

In #310 the exception message was replaced by the backtrace. We can add the exception message back at a different in the check response key and display that (and on user request in the UI show the full backtrace or vice versa)

[laceyallyn]

The benefit of this is that we display minimal details by default (reducing strain) but for the cases, where the error message provides little to no value without the line number, we can support them by displaying the entire backtrace.

Will require UI work in the future.

[mohabusama]

Can be fixed by #399