Change filter-stack semantics such that they don't always narrow

(but still stack)
keybinding for "filter everything"
turn the error message into a displayable string
Sanitize the exceptions from exile so they make it back to the main process

(Otherwise you'd get exceptions printed by the subprocess saying that there
were pickling errors caused by ctypes pointers (which, according to
experimentation, were not in the exceptions themselves but presumably in
the frames held by the traceback.))