Can some error results be merged or replaced with panics?

[fjarri]

At the moment umbral is designed to never panic and always return a Result/Option. In some cases this results in rather silly errors that, if happen, are not something one can recover from, but rather indicate a bug in the rust-umbral, or the calling code. Perhaps we can introduce some panics to simplify the API. Although we need to remember that rust-umbral may be run in embedded environments.

Additional reading:
https://doc.rust-lang.org/book/ch09-03-to-panic-or-not-to-panic.html
https://www.unwoundstack.com/blog/rust-error-handling.html

A (perhaps non-exhaustive) list of candidates for replacement with panic:

• OpenReencryptedError::NoCapsuleFrags - the caller should ensure that the list of cfrags is not empty
• OpenReencryptedError::ZeroHash - very low probability of happening either randomly or intentionally (caused by some malicious actor)
• SecretKeyFactoryError::ZeroHash - see above.
• EncryptionError::PlaintextTooLarge - this is basically an OOM situation. In all other cases of OOM the code will panic anyway. For embedded environments we may provide an infallible encrypt_to_buffer or something.

Some errors may be merged, since they are effectively the same from the user's point of view:

• IncorrectKeyFragSignature and IncorrectReencryption in CapsuleFragVerificationError
• CiphertextTooShort and AuthenticationFailed in DecryptionError
• IncorrectCommitment and IncorrectSignature in KeyFragVerificationError. Possibly merged with DelegatingKeyNotProvided and ReceivingKeyNotProvided too.