feature: limit printed traceback lines

[Fak3]

If handler backtrace is True, the output traceback is very long. Especially for async functions - more than a hundred of lines come from the event loop, and clutter the output (example: https://gist.github.com/Fak3/81cf8554414e2d82cfcad93294f3e981)

It would be nice to have a way to limit number of backtrace lines printed, like sys.tracebacklimit = 3 does.

[Delgan]

In the next v0.3.0 release, you should be able to set backtrace=False to reduce the size of the traceback, a new parameter diagnose will allow to independently control whether or not the variable values should be displayed. 👍

I was not aware of sys.tracebacklimit, I think the printed exceptions should actually make use of this value.

[Delgan]

Starting from v0.3.0, the exception formatting will rely on the sys.tracebacklimit variable to determine the depth of the stacktrace to display.

Also, note that adding an handler with backtrace=False should drastically reduce the number of frames displayed.