Fix: run:inside hangs (doesn't error) when no/invalid SSH key

[justinwilaby]

Fixes the hanging process when no SSH keys or invalid SSH keys are used to run a process inside a dyno

testing

1. Move all keys from the default location ~/.ssh to another directory
2. create another key ssh-keygen
3. Don't upload it to heroku
4. run inside `./bin run run:inside -a YOUR_APP YOUR_DYNO "node -v"
5. OBSERVE: The Permission denied (publickey) error is thrown and the process exits with exitcode 1

[angeldcampbell]

🪵

[k80bowman]

I wouldn't mind a test or two around this, but the implementation looks good. Thank you.