Error backtraces for debugging tests

[Stebalien]

The error crate we use, anyhow, supports backtraces. However, we can't use them right now for a few reasons:

1. We tend to extend errors turning them into strings then constructing a new anyhow error with some context. The correct approach is to use my_anyhow_error.context (and friends).
2. When we have to turn an anyhow error into a string, we should prefer {:?} where possible, or maybe just when debugging? {} will only include the last context, {:#} will include all context but no backtrace, {:?} will include everything.