mdbook should exit if serve can't serve

[joshrotenberg]

If, for example, the address is already in use, an error is given but the tool doesn't exit:

❯ mdbook serve                                                                                                130 ↵
2021-05-24 13:23:22 [INFO] (mdbook::book): Book building has started
2021-05-24 13:23:22 [INFO] (mdbook::book): Running the html backend
2021-05-24 13:23:22 [INFO] (mdbook::cmd::serve): Serving on: http://localhost:3000
thread '<unnamed>' panicked at 'error binding to [::1]:3000: error creating server listener: Address already in use (os error 48)', /Users/joshrotenberg/.cargo/registry/src/github.1485827954.workers.dev-1ecc6299db9ec823/warp-0.2.5/src/server.rs:213:27
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace
2021-05-24 13:23:22 [INFO] (mdbook::cmd::watch): Listening for changes...

[joshrotenberg]

Potentially related issues: #1472 and #1321