POC: New beresp.error VCL variable

[dridi]

Following several VDD discussions, this patch series shows how getting a backend error could look like in VCL, and how it could be articulated in the core code.

This should check the boxes from the last consensus:

• a plain text error message
• a new VCL variable to get the error (beresp.error)
• VCL constants for safe comparisons (error.*)

The first two commits are unrelated, I found a bug in libvcc while I was working on this. I will submit a pull request once I'm done with that bug (only partly fixed here) so the relevant commits to review are the ones starting with "POC".

If we still have consensus on this approach, I will find someone™ to submit something comprehensive.

[dridi]

Bugwash summary:

• Turn errors into a struct vrt_error { const char *type, *msg };
  • @nigoroll wanted to categorize errors (example: "backend timeout")
  • I suggested an (ERROR).type VCL attribute
  • It coud also be a list of tags (not brought up during bugwash)
• Turn beresp.error into a proper ERROR
• Teach $Error to vmods (add vmod_debug coverage)
• Error messages from the documentation should be logged (I meant to initially and forgot)

Then we reevaluate the error reporting.

[dridi]

I wanted to revisit this ticket this morning and noticed that I forgot about the libvcc bug. I pushed 0c8448e0ef...b77b709510 to fix it.

[nigoroll]

bugwash label removed because this ticket needs an update