final void failRequest(Throwable cause) {
        if (id() >= 0) {
            failAndReset(cause);
        } else {
            // No need to send RST because we didn't send any packet.
            fail(UnprocessedRequestException.of(cause));
        }
    }
Writes the {@link HttpHeaders trailers} to the {@link Channel}.
Note that the written data is not flushed by this method. The caller should explicitly call
{@link Channel#flush()} when each write unit is done.
core/src/main/java/com/linecorp/armeria/client/AbstractHttpRequestHandler.java