Split time too old error into two distinct errors.

This commit splits the two rule validation errors related to the timestamp
being too old into two distince errors rather than grouping them under the
same one.  Thus there is now a new ErrCheckpointTimeTooNew in addition to
the existing ErrTimeTooNew error.

This allows the caller to detect the when a block is rejected due to a
time-related checkpoint failure whereas before the combined error did not.