Rework API for failure/success signaling

[lucab]

Currently there are several parts of rust-amqp where errors are being sunk, due to API not providing return values to signal errors/success. In order to be fixed, this requires changing existing API to signal all possible errors.

Troubling methods spotted so far:

• [x] Channel::close()
• [x] Channel::write()
• [x] Channel::send_method_frame()
• [ ] most of Basic

(More will follow after further inspection)

[Antti]

Fixed all channel related ones, fixed most of the Basic, only Iterator implementation left.