uloop: Add flag to allow callback to be called on error conditions.

uloop: Add flag to allow callback to be called on error conditions.

In some conditions, an application is interested in errors happening
on a file descriptor and might be able to resolve the issue in the
callback function.

This patch adds a flag to notify the uloop framework that errors
should be passed to the callback function, instead of silently
removing the fd from the polling set.

Signed-off-by: Karl Vogel <karl.vogel@gmail.com>