Wasmtime should always print guest panics

#### Feature

Currently, Wasmtime doesn't always print anything to `stderr` when the guest panics: e.g. when serving incoming HTTP requests, panics are only printed if they happen before a response is sent. If the panic happens after the response headers have been sent, the panic is silently swallowed, and will only show up with the right `WASMTIME_LOG` incantation.

#### Benefit

Make guest panics easier to identify and debug.

#### Implementation

I'm not sure where this needs to be added, unfortunately.

#### Alternatives

Do nothing.