Warn about almost expired timeouts

[pohlt]

If you specify a timeout and the action eventually worked but was close to timeout (e.g 80% of the timeout period passed), a warning would be helpful for the user. She/he could then increase the timeout or investigate the slow operation. The fraction of passed timeout for a warning could be configurable.

[hynek]

Not sure how that should work mechanically? You can approximate this by using the iterator-based approach and look at the Attempt object?