Just use this type as return error type for your handler functions (ex: Result<String, custom_derive::Error>)
and after running the usual let _guard = sentry::init(...);
and it will send any error to sentry.
Note that you should use the failure crate to propagate your errors if you wish to have proper backtrace handling.
Also, using sentry_rocket::sentry instead of sentry will ensure the versions match so that everything works properly
(also, then you don't need sentry as a dependency for your project, only sentry_rocket)