#230 generic handling of bgerror

[David N. Welton]

As per:

http://sourceforge.net/mailarchive/forum.php?thread_id=1196493&forum_id=6718

There needs to be a way, if possible, for tcllib
packages to cleanly handle errors in the background.
The problem is that no library package should really be
setting the bgerror command, yet, at the same time,
they should be responsible for not generating these
errors if possible.

In some cases, it may be possible to eliminate possible
sources of unexpected errors, such as file input/output
by making sure they do not generate errors.

In the future, maybe it's worth extending Tcl so that
seperate bgerrors can be registered for different
things? I don't know how this might work.