Improve error messages for retrieval and decryption

[piotr-roslaniec]

Currently, if during Bob.retrieveAndDecrypt we fail to retrieve a minimal amount of cFrags, we throw the following error:

Not enough cFrags retrieved to open capsule Capsule:xxxxxxxxxxxxxx. Was the policy revoked?

However, there are many different reasons why this could happen:

• Policy was revoked
• Policy was created, but not enacted (see delayed enact flow)
• Ursulas fail to provide Porter with cFrags

How should we address these scenarios?

• Policy was revoked
  • Currently not implemented, remove revocation from the error message.
  • In the future, check with the SubscriptionManager contract.
• Policy was created, but not enacted
  • Check with the SubscriptionManager contract? That would require a web3 provider for Bob.
• Ursulas fail to provide Porter with enough cFrags
  • Detect this issue early and throw a relevant error.

Ordering of these checks will be significant.