New tool handling type

[Phil Norman]

Hi.

Just pushed a little change to the tool handling. I've added a new prefix, "|?". It's intended as a variant on "|", but with better error handling.

If the tool exits successfully and prints nothing to stderr, then "|?" acts just like "|". However, if the tool exits with some failure code, the text buffer is left as it was without replacing it with (usually) the error message. Additionally, anything that comes out of stderr is sent to an Errors window, rather than being inserted into the buffer alongside stdout.

This fixes a bunch of annoyances I've had for a while.

Enjoy,

Phil