CV errors should log to a separate file as well as cv.log on worker boxes

[julianmorley]

When a CV error is detected by prescat, that error is currently logged only to the current cv.log. The problem is that this log is regularly compacted and deleted on a 7-day schedule, which results in CV errors 'cycling out' of the logs directory, and thus becoming hard to find.

These errors should log to cv_error.log either as well as, or instead of, logging to cv.log. The cv_error.log should not be log-rotated so that it persists for more than 7 days, and so that we can instrument our monitoring systems to watch that log for new errors.