LP#1526185 egNet PERM_FAILURE handler

LP#1526185 egNet PERM_FAILURE handler

Support a new egNet.handlePermFailure() callback. When this function is
defined, it's invoked any time an egNet request returns a PERM_FAILURE
event.

When invoked, the in-progress promise linked to the original
request is effectively superseded by the promise returned by the
egNet.handlePermFailure(), living and dying with the new promise. This
makes it possible to hold off resolving the original request until the
handler is done.

Signed-off-by: Bill Erickson <berickxx@gmail.com>