Handle case where default browser can't be found or launched

[kitplummer]

E.g. on a linux console with no desktop

[kitplummer]

❯ cargo run login
    Finished dev [unoptimized + debuginfo] target(s) in 0.10s
     Running `target/debug/tdi login`
tdi: authenticating, a browser window will open.
thread 'main' panicked at 'called `Result::unwrap()` on an `Err` value: Io(Os { code: 2, kind: NotFound, message: "No such file or directory" })', src/auth.rs:131:44
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace