Using `sys.excepthook` to silence the must-use warning has some false
negatives: applications may catch the exception and then quit
normally, e.g. becaue the error is well known and does not require
a traceback to be shown (which would be noisy). The current
implementation prints even more noise in that case.
In addition to the existing heuristic, silence the warning if
*nothing* has been elaborated, which is almost always a reliable
sign. It doesn't work if multiple designs are independently created
in the application and some of them are dropped without being used,
but this is unavoidable as it is not distinguishable from the mistake
this warning is attempting to prevent.
Fixes#848.
