Refactor Error class

Backtraces have sadly been broken from some time, and it was a major
pain to find the source (still searching). This commit removes all the
self rolled code with the backward-cpp library

This library features a much more comprehensive set of options, pretty
printing and deals with dependencies for us.
25 jobs for !225 with df/replace-error-backtrace-cpp in 15 minutes and 17 seconds (queued for 5 seconds)
merge request