Document client errors

[alexkalish]

I recently stumbled across the long list of error codes in the authn client here.

Two observations:

1. These don't appear to all be errors. Some clearly are (e.g. "CAKC001 Error opening the access token file"), while others are simple informative log messages (e.g. "CAKC036 Logged in").
2. Only two of these errors is actually documented in our official docs.

It's wonderful that we have so many known error conditions, but we could provide more benefit to users by fully documenting them.

AC:

• Differentiate errors messages from informative log messages in the afore mentioned source file. Comments are fine. Mostly it should be obvious which are which, without having to look at use of the constant.
• Minimally, document the root cause or trigger of each error so that they can go into official documentation. When possible, provide details for resolving/mitigating the error.
• File a docs ticket to update the official docs with the info above. This content would be placed in the errors table referenced above.