Add a command-line option to always print full stack traces in failure report

[vkarak]

This is quite useful for debugging.

[vkarak]

This will become a special case of #650 if we print always print stack traces at the debug level.

[vkarak]

I suggest printing stack traces in verbose mode, because otherwise the user would have to cope with the whole debug output, which is a lot.

[vkarak]

The implementation of this is not so straightforward as it sounds. This issue refers to stack trace printouts inside the failure report (I updated the title of the issue). Currently the failure report and the exception formatting are handled by functions that return the formatted strings, which then are logged at info level. Perhaps the report and exception formatting should be printed/logged instead of being returned as a string.

[vkarak]

This should be now straightforward to implement, since the verbosity level can be accessed by anywhere in the framework. I will schedule it for one of the upcoming releases.

[vkarak]

We don't need a new option for that. Increasing verbosity should do the trick.