toolbox.sh: Suppress errors in command_not_found_handle()

toolbox.sh: Suppress errors in command_not_found_handle()

When the user enters an unknown command, or a script contains an
unknown command, the command_not_found_handle() function provides
suggestions if the unknown command appears to be in a module.
However, because the function does not suppress errors when looking
for modules in the module search paths, it emits errors if any of
the search paths do not exist.

This commit makes the command_not_found_handle() function suppress
errors if any of the module search paths do not exist.