Record and report timestamp of last I/O error

The cvmfs client records and reports the number of I/O errors for monitoring. There is no way, however, to see how far in the past the I/O errors took place. In order for monitoring systems to be able to clear errors, it would be useful if cvmfs reported the timestamp of the most recent I/O error, too.

Along with this change, we should adjust the Nagios probe so that it can ignore I/O errors older then X hours/days.