gh-274: Handle blocking I/O errors in GreenSocket
gh-274: Handle blocking I/O errors in GreenSocket

Fix recv_into(), recvfrom(), recvfrom_into() and sendto() methods of
GreenSocket to handle blocking I/O errors (ex: BlockingIOError on
Python 3). Even if the trampoline was called, the socket method can
still fails with an I/O errors for various reasons (see manual pages
of the C functions for examples).

Ref: https://github.com/eventlet/eventlet/issues/274

collinstocks/eventlet,collinstocks/eventlet