[core/engine] handle single-module errors
[core/engine] handle single-module errors

instead of terminating the whole status bar when an error occurs, just
show a (truncated) error for that single widget.

this should also enable auto-recovery if the module returns to a "good"
state, but that hasn't been tested yet.

see #221 and #222

tobi-wan-kenobi/bumblebee-status,tobi-wan-kenobi/bumblebee-status