Store failed builds in the cache

Description

Changes proposed in this merge request:

• Add metadata for the result of a build.
• Store failed build artifacts in the cache unconditionally.
• Fix up call sites where it is assumed that if it is cached it was successful.
• Skip straight to failing a build if it is a cached fail.
• Permit manual retrying by flagging that it's already skipped the build once so if it's queued again it rebuilds.

This merge request, when approved, will close issue/bug: #76 (closed)

CHANGELOG/Release Notes

• Made it push artifacts. --on-error=continue was only missing putting the artifact in done so it could be queued for push. --on-error=quit was more difficult and may not have an adequate solution.
• Fixed after initial review and tidied the history. #76 (comment 74605228) yet to be addressed.
• Rebased and code tidied. #76 (comment 74605228) yet to be addressed and history tidied.

buildstream/_frontend/widget.py

@@ -369,7 +369,7 @@ class LogLine(Widget):
             if consistency == Consistency.INCONSISTENT:
                 line = p.fmt_subst(line, 'state', "no reference", fg='red')
             else:
-                if element._cached() and not element.get_public_data("result")["success"]:
+                if element._cached() and not (element.get_public_data("result") or {}).get("success", True):
                     line = p.fmt_subst(line, 'state', "failed", fg='red')
                 elif element._cached():
                     line = p.fmt_subst(line, 'state', "cached", fg='magenta')