Move logging errors into validation warnings for Add Trips / TripSchedule creation

[trevorgerhardt]

The logging errors found here: https://github.com/conveyal/r5/blob/dev/src/main/java/com/conveyal/r5/transit/TripSchedule.java#L84-L106

Should be checked when validating a modification so that the errors can be more clear for the user. If TripSchedule returns null it is likely to cause an NPE for the user without information for how to fix it.