[tcp] Check API for User Errors at Lower Level

[BrianZill]

We don't have a consistent place to check API calls for user errors. For example, in the send path, there is a check that the user isn't attempting to send an absurd amount of data in a single send. This check isn't performed until we're all the way up in Sender::send(). This presumably could/should be done at a lower level, perhaps even in the protocol-independent part of the API code.