Return result instead of panic when returning an `installation` Octocrab instance

[krishmas]

octocrab/src/lib.rs

Line 981 in 7e280be

panic!("Github App authorization is required to target an installation");

Would be nicer to return an Error result (like other functions that return different types of Octocrabs do) instead of panicking so the application using doesn't immediately crash out

[XAMPPRocky]

Thank you for your issue! A PR adding this would be welcome 🙂