feat(sdk): Decrypt error is StreamrClientError

[teogeb]

Removed custom DecryptError class. Using StreamrClientError with code DECRYPT_ERROR instead.

Changed some wording of decrypt errors: encryption keys are no longer referred as group key, which is an internal term for encryption key (see e.g. exports.ts#36).

Cause parameter

Considered adding cause parameter for StreamrClientError (https://developer.mozilla.org/en-US/docs/Web/JavaScript/Reference/Global_Objects/Error/cause). Decided not to include it as it we've earlier had compatibility issues with it when using Firefox. Maybe browser support is nowadays better. We can add that later if needed.

The cause parameter would link root cause Error instances to StreamrClientError and therefore provide very detailed stack traces. For client errors such stack traces are maybe not needed in practice. I.e. if decryptWithAES() throws an error, we'll create a StreamrClientError with message of "AES decryption failed" and type of DECRYPT_ERROR. In most cases these bits of information are sufficient for analyzing application level issues.

Future improvements

• convert all other custom errors to StreamrClientError
• add toEqualStreamrError() test utility