unit: move "not supported" check after condition check in unit_start()

unit: move "not supported" check after condition check in unit_start()

Make sure we always check conditions before checking whether the unit
type is supported in unit_start(), since condition checks are "clean
errors", while "not supported" errors are fatal.

This cleans up the boot output of systemd in containers, where a lot of
NOTSUPP lines were shown befor this fix.

This partially reverts 8ff4d2ab0d4758e914aea6d86154d85f2b2c787f which
reorder the checks.