feature request: when external tool fails print its stderr

[GuilhemBichot]

Hello. I know someone who had this bzr alias in bazaar.conf:
diff = diff --diff-options=-p
This gives him -p-style text diffs, all fine.
Then he forgets about this alias' existence, and much later does "bzr diff --using=kdiff3". This resolves to
"bzr diff --diff-options=-p --using=kdiff3"; difftools thus calls "kdiff3 -p" which fails (kdiff3 has no -p option); but as difftool.py hides the stderr output of kdiff3 (redirects it to a temp log), there is no output at all:
bzr diff --using=kdiff3
shows no output or message, whereas "bzr diff" shows a diff. Gave the impression that difftools had a bug.
Ok, doing "echo $?" showed 1, but this is not so intuitive.
How about, if the diff tool returns !=0, printing the content of the temp log?