Specifying nonexistent package name results in "successful" no-op run

[ijackson]

Hi. Thanks for this tool which we rely on heavily for our releases. I have an infelicity to report:

To reproduce

git clone https://gitlab.torproject.org/tpo/core/arti
cd arti
git checkout arti-v1.1.13
cargo --offline set-version --bump patch -p this-package-does-not-exist
echo $?

Expected behavour

An error message, and a non-zero exit status.

Actual behaviour

No output, no change to the tree, and a zero exit status.