Make Thema errors excellent: clear, actionable, informative about thema

[frossano-grafana]

this is a usability issues, not a stability issue
We need to deal with validation error first, and then with the messages for authors when we start making this used in Plugin generation

[sdboyer]

#82 begins the use of github.com/cockroachdb/errors. However, there's a fair bit of planning and design on its own that needs to go into adopting the library - see this whole design doc that covers the behaviors.

i could be talked out of it, but i think for the role we would like to see Thema occupy - a highly portable utility that can tuck into individual programs or complex operational topologies - ensuring our errors have the properties outlined in that document's motivation section seems like the right move. i'm fine with a complex errors implementation inside thema, so long as it's simple to use on the outside.

[frossano-grafana]

I agree there is a fair bit of planning and design to do, and this is one of the things I talked abut with all the designers we interviewed. Error handling is both a technical and a usability issue, and we need to start from what we want to expose to final users and walk back to what we can achieve based on constraints in the implementation (there might be none, but there might be some big ones)
Motivation behind the errors is super important, and for me in the end it boils down to: what do we want users to do when a specific error (or class of errors) comes out?

[sdboyer]

Closing in favor of #164