This solves #5679 , and also implements #5515, #5509 and lays the ground for #5516
With the introduction of Wrap, Is and As in the standard library, we've now got built-in support for wrapping.
On top of that, a common pattern in the community is to define errors tailored to the context of each project while still conforming to the error and Unwrap interfaces (see Upspin, Chromium)
The output now includes stack traces and additional info
this change prevents errors from being automatically cleared once they are read. A new flag `-flush` is introduced to flush errors on read if necessary.
This is a temporary mitigation for the issue described in #3065.
The intent is to merge this, cut a 4.6.1 release, and then come up with
a more comprehensive solution for 4.7.0.
