Allow loading a schedule with no matches.

While not really useful, it's a valid state.

Change-Id: Ifb9d1e8b3f80b743ce45228a93cf20751ef3eadf
Make the more-matches-than-time issue not an error.

It's still reported in the same way, but don't include it in the
error count. This allows an overrunning schedule to be valid.

Change-Id: Icc25f3e23b5516edbd6ce4d9586d3d9ef7790775
Also warn about empty schedules.

Change-Id: I87c2945b8f5421720961fb6eec9eabbe049aa4ff