Consider renaming fail_map to error_map

[mre]

We no longer use the term failure in our system; we now refer to it as error. Therefore, it makes sense to rename fail_map to error_map for consistency. Similar to #1443, this will introduce a breaking change, but since we haven't reached a stable version yet, I want to address these naming inconsistencies beforehand.

[mre]

Related discussion, which could be tackled at the same time: #1013

[thomas-zahner]

I totally agree with that change 👍

As for #1013 this was actually already implemented in 250f7a8. Although, I think that the text field is a bit verbose, maybe we could reduce to just the HTTP status text.

The current JSON output looks like:

  "success_map": {
    "http://www.example.com/": [
      {
        "url": "https://www.iana.org/domains/example",
        "status": {
          "text": "OK (200 OK)",
          "code": 200
        }
      }
    ]
  },

[mre]

Yup, sounds like a good idea!