Impossible to re-register a previously whitelisted candidate #1605

Once a whitelisted candidate is unwhitelisted & unregistered it's impossible to whitelist & re-register them again, the only way to get them back into the candidate list is to allow the DPOS contract to deduct the 1.25M LOOM registration requirement.

Reproduction steps:

1. loom dpos3 whitelist-candidate A
2. loom dpos3 register-candidate A
3. Wait a couple of election cycles.
4. loom dpos3 remove-whitelisted-candidate A
5. loom dpos3 unregister-candidate A
6. Wait a couple of election cycles.
7. Now try re-registering A:
   • loom dpos3 whitelist-candidate A - nope, Cannot whitelist an already whitelisted candidate.
   • loom dpos3 change-whitelist-info A - no error, but doesn't actually do anything since this only works on registered candidates.
   • loom dpos3 register-candidate A - attempts to transfer 1.25M LOOM to DPOS contract since the whitelist amount is still zero.