This doesn't split apart the current error type into one that tracks
location and one that doesn't, although that might be easier to do once
we have this.
Instead, this is primarily intended to support custom error types that
lazily materialize the error message in case that can be avoided by
completely handling the error. For example, many file system operations
are *expected* to produce errors even in the hot path and we don't want
to render `ENOENT` (for example) to a pretty string and instead will
directly query the error to understand and handle it in code.
The type parameter ordering isn't the most obvious, but helpfully allows
us to default the error type in a useful way.
---------
Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Chandler CarruthandJon Ross-Perkins
e99448eecf