deref of an empty leafref reports further validation failures #2592

Now that #2584 is fixed, libyang produces "surprising" diagnostics when deref() fails the require-instance true. Here's a small example:

module deref-arith {
  yang-version 1.1;
  namespace "urn:example:deref-arith";
  prefix da;

  list band {
    key name;
    leaf name { type string; }
    leaf lower { type uint32; mandatory true; }
    leaf upper { type uint32; mandatory true; }
  }

  list channel {
    key band;
    leaf band {
      type leafref { path "/band/name"; }
    }
    must "deref(band)/../upper - deref(band)/../lower >= 50" {
      error-message "Band is narrower than 50";
    }
  }
}

{
  "deref-arith:band": [
    { "name": "wide", "lower": 100, "upper": 200 }
  ],
  "deref-arith:channel": [
    { "band": "wide" },
    { "band": "missing" }
  ]
}

Here's how it fails. The first error message is expected, the second is, in my opinion, a superfluous one:

$ yanglint -t config deref-arith.yang data.json
libyang err : Invalid leafref value "missing" - no target instance "/band/name" with the same value. (/deref-arith:channel[band='missing']/band)
libyang err : Band is narrower than 50 (/deref-arith:channel[band='missing'])
YANGLINT[E]: Failed to parse input data file "data.json".

We bumped into this in a unit test of our internal project.

I wonder if it's possible (and standard-compliant) to return a better error message here. Please note that my target audience is not the author of the YANG model or someone writing the YANG data in JSON/XML/..., but someone who works with a CLI, who just made a typo in a leafref name, and who is confused by an error message complaining about a "too narrow channel".