Create FAQ for common errors on encrypt/decrypt

[seebees]

Errors can bubble up from the encrypt/decrypt functions.
It would be nice to have clear explanations for common errors.

In creating this documentation,
the error messages can also be re-evaluated for clarity.

See #152 as an example.

[holmser]

"unencryptedDataKey has not been set" and "Noop keyring is not allowed: Set a keyId or discovery" are the two that have been giving me trouble so far.

[monscamus]

Definitely backing @holmser on those clearer error messages - also in related business when the IAM permissions aren't quite right it would be nice to get a message that says insufficient IAM privileges rather than the cryptic null parameter not allowed which I think you tend to get at the moment.

[metalcamp]

I get following error: "unencryptedDataKey has not been set" during decryption as well.

Following example here: https://github.com/aws/aws-encryption-sdk-javascript/blob/master/modules/example-node/src/aes_simple.ts using Raw AES keyring in example here.

Only difference being I split up encryption and decryption steps into separate functions.

What am I missing? Is KMS required even if explicitly want to avoid it, for example using Hashicorp Vault for storing secrets?