Error while polling connection prints to stderr

[evie-calico]

The eprintln responsible is here:

fastwebsockets/src/handshake.rs

Line 101 in d1dc277

eprintln!("Error polling connection: {}", e);

typically libraries use a tracing/log feature for internal error messages so that consumers can opt in, but i think returning the result here would be the best solution. I can make a PR for whichever of these is preferred if it'd help.