Common juju errors and functions to annotate errors

 The juju/errors library provides an easy way to annotate errors without
 losing the original error context.
 .
 The exported 'New' and 'Errorf' functions are designed to replace the
 'errors.New' and 'fmt.Errorf' functions respectively. The same underlying
 error is there, but the package also records the location at which the
 error was created.
 .
 A primary use case for this library is to add extra context any time an error
 is returned from a function.