clarifies sessions -c command

Don't land this just yet, it'll just cause merge conflicts with #4063.  I'll rebase it after I land that.

This PR clarifies the sessions -c usage info and changes the session execute RequestError rescue to be more specific and informative

## Verification

Don't do this yet, but...

- [ ] Get some sessions, preferably a shell and any native win meterpreter
- [ ] sessions -i ID -c calc.exe
- [ ] success on both sessions
- [ ] sessions -i ID -c pwd
- [ ] get: pwd is not a valid shell command or binary path