The bgerror command doesn't exist as built-in part of Tcl. Instead,
individual applications or users can define a bgerror
command (e.g. as a Tcl procedure) if they wish to handle background
errors.
A background error is one that occurs in an event handler or some
other command that didn't originate with the application.
For example, if an error occurs while executing a command specified
with the after command, then it is a background error.
For a non-background error, the error can simply be returned up
through nested Tcl command evaluations until it reaches the top-level
code in the application; then the application can report the error
in whatever way it wishes.
When a background error occurs, the unwinding ends in
the Tcl library and there is no obvious way for Tcl to report
the error.
When Tcl detects a background error, it saves information about the
error and invokes the bgerror command later as an idle event handler.
Before invoking bgerror, Tcl restores the errorInfo
and errorCode variables to their values at the time the
error occurred, then it invokes bgerror with
the error message as its only argument.
Tcl assumes that the application has implemented the bgerror
command, and that the command will report the error in a way that
makes sense for the application. Tcl will ignore any result returned
by the bgerror command as long as no error is generated.
If another Tcl error occurs within the bgerror command
(for example, because no bgerror command has been defined)
then Tcl reports the error itself by writing a message to stderr.
If several background errors accumulate before bgerror
is invoked to process them, bgerror will be invoked once
for each error, in the order they occurred.
However, if bgerror returns with a break exception, then
any remaining errors are skipped without calling bgerror.
Tcl has no default implementation for bgerror.
However, in applications using Tk there is a default
bgerror procedure
which posts a dialog box containing
the error message and offers the user a chance to see a stack
trace showing where the error occurred.
