Improve HTTP error handling

[thrawn01]

Purpose

To improve how we report errors when making HTTP calls to other groupcache instances. See #59 for details on the proposal.

Implementation

• Added groupcache.ErrNotFound which should be returned from an implementation of GetterFunc to indicate the requested value is not available
• Added groupcache.ErrRemoteCall which is returned from group.Get() when an error that is not a ErrNotFound is returned during a remote HTTP instance call

[ghost]

Probably want to add an entry to the CHANGELOG for this as well?