Give more descriptive error messages to encrypt() and decrypt()

[habics]

For example when using a 2048 bits key and call decrypt() on a ciphertext that more than 256 bytes long, we get a generic error message that says Err value: Decryption a more appropriate message would also tell that the cipher text is too long for this key.

Thank you.

[tarcieri]

The reason it's a bit scary to add different types of decryption errors is because the information sidechannel they introduce can potentially be leveraged by an attacker. Example: https://en.wikipedia.org/wiki/Padding_oracle_attack

A modulus size mismatch for the ciphertext is probably ok. We could potentially introduce a new e.g. CiphertextSize variant to Error which shouldn't leak any information useful to the attacker since the only property of the private key it relies on is the public modulus.