fastboot: fix SocketMock send failures.

Fixes SocketMock::ExpectSendFailure() to allow unit testing of errors
during send, and adds tests for ExpectSendFailure() and
AddReceiveFailure().

Also adds missing tests to make sure ReceiveAll() continues to read
until failure or all bytes have been read.

Bug: http://b/26157893
Change-Id: I67e7d6de8e8ec4a3b62a6b7d7217f7530862edf7