Fixes per waigani review. Improve error handling.

Use errors.Errorf instead of fmt.Errorf.
Warn on connection close error in api.Open.
godoc comment coverage & fixes.
Remove redundant typenames from slice value.
Fixes per waigani review. errors.Trace all the things.

Plus minor changes to make params.ErrCode and admin root tests
compatible with errors.Trace.
Fixes per waigani review. Stutter less.

Remove redundancies from symbol names, taking the imported package name
into account.