Standardize snap-related errors

We should standardize all common snap-related errors. This means that we should choose error codes and standard messages, and probably add them to our [`eth-rpc-errors`](https://github.com/MetaMask/eth-rpc-errors) library. If we can reuse existing error standards, we should do so.

Before we start an implementation, a proposal should be drafted (in written form, anywhere) of the different kinds of errors that exist and what error codes and messages they should have.

Start by cataloging the common snap-related errors and reviewing the eth-rpc-errors library for reusable standards. Done means producing a written proposal that defines the error categories, codes, and standard messages before implementation begins.

Written by the indexing model from the issue text.