clippy: actually fail if there are any warnings
So far, only errors would trip the CI, this makes it so all warnings are
elevated to errors, making it a CI fail if any warnings are present.
This was the intended behavior, I just missed this when adding clippy.
#require execbit
$ hg init repo
$ cd repo
$ echo foo > foo
$ chmod 644 foo
$ hg ci -qAm '644'
$ chmod 755 foo
$ hg ci -qAm '755'
reverting to rev 0
$ hg revert -a -r 0
reverting foo
$ hg st
M foo
$ hg diff --git
diff --git a/foo b/foo
old mode 100755
new mode 100644
$ cd ..