Document known non-exceptional errors

[corrideat]

Problem

Sometimes, error conditions happen that, despite looking like errors or being logged like errors, are, in fact, expected to happen, or least not severe. One such example is that a decryption error generally isn't treated like a fatal condition.

Solution

Document all known instances of this as they become known, including:

1. A brief description of the error
2. Why it happens
3. Why it's not really an error
4. If applicable, under which situations it would be an error