Errors on arg read/write should not block channel shutdown

[prashantv]

If a ArgReader is closed without reading all data, it returns an error:

tchannel-go/fragmenting_reader.go

Line 200 in a815a8e

r.err = errMoreDataInArgument

This does not close the underlying message exchange, so as far as the channel is concerned, there's a pending call. However, this pending call can never be cleared, since all further operations return the same error.

We should close the message exchange on any errors,

tchannel-go/mex.go

Line 235 in a815a8e

func (mex *messageExchange) shutdown() {

That will ensure that the channel close isn't blocked.