Drop an error condition unrelated to mongo.
Add a note about what io.EOF means.
Drop "connection is shut down" as well.
Drop the two related tests.
Factor out checkConnectionFailure.
Check for various errnos.
Merge pull request #1276 from ericsnowcurrently/replicaset-IsReady-drop-condition

Drop an error condition unrelated to mongo.

The motivating error actually came from the API client, not mongo.  So leaving the check in might lead a future reader to erroneously believe mongo might produce such an error (or that isConnectionNotAvailable should handle it).

(Review request: http://reviews.vapour.ws/r/591/)