Add option for whether tracebacks should be shown in 5xx responses

[jwodder]

Currently, 5xx responses contain an error traceback in their bodies in order to aid in debugging, but this is bad practice. This behavior should be disabled by default, and a CLI option (--serve-errors?) should be required to re-enable it.

• When serving errors, panics should also be converted to 5xx responses (ideally containing a backtrace) using https://docs.rs/tower-http/latest/tower_http/catch_panic/index.html.