Make it easier to debug date parsing errors

When message dates failed to parse, the error displayed would try to
include the time object it failed to obtain, which would display as
something like 0001-01-01 00:00:00 UTC, which isn't of much help.

Instead, display the text we were trying to parse into a date, which
makes the problem easier to debug.

worker/lib/parse.go

@@ -178,7 +178,7 @@ func parseDate(h *mail.Header) (time.Time, error) {
 			return t, nil
 		}
 	}
-	return time.Time{}, fmt.Errorf("unrecognized date format: %s", t)
+	return time.Time{}, fmt.Errorf("unrecognized date format: %s", text)
 }
 
 func parseAddressList(h *mail.Header, key string) ([]*models.Address, error) {