Trigger exceptionCaught() when VoidChannelPromise fails

Related: #3190

Motivation:

When an outbound handler method raises an exception, its promise is
marked as failed.  If the promise is done already, the exception is
logged.

When the promise is void, exceptionCaught() must be triggered to notify
a user. However, AbstractChannelHandlerContext simply swallows it.

Modifications:

Do not swallow an exception when the promise is void.

Result:

A user who uses a void promise for an outbound operation will be
notified on failure.