libexpr: print value of what is attempted to be called as function

[Ma27]

Motivation

Depends on #9753

This basically prints out what was attempted to be called as function,
i.e.

map (import {}) [ 1 2 3 ]

now gives the following error message:

error:
       … while calling the 'map' builtin
         at «string»:1:1:
            1| map (import <nixpkgs> {}) [ 1 2 3 ]
             | ^

       … while evaluating the first argument passed to builtins.map

       error: expected a function but found a set: { _type = "pkgs"; AAAAAASomeThingsFailToEvaluate = «thunk»; AMB-plugins = «thunk»; ArchiSteamFarm = «thunk»; BeatSaberModManager = «thunk»; CHOWTapeModel = «thunk»; ChowCentaur = «thunk»; ChowKick = «thunk»; ChowPhaser = «thunk»; CoinMP = «thunk»;  «18783 attributes elided»}

Remains a draft until #9753 (the parent of this PR) is merged.

Context

Low-hanging fruit in the spirit of #9753 and #9754 (means 9999years did
all the hard work already), but for

attempt to call something which is not a function but ...

Priorities and Process

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[cole-h]

(very minor thing, but there is a dpulls action on this repository, so if you write Depends on #9753 in the PR body, the dpulls action should fail until that PR is merged, and then post a comment once that's happened, if you're interested)

[dpulls]

🎉 All dependencies have been resolved !

[Ma27]

Rebased onto latest master with #9753 being resolved.
cc @roberth

[roberth]

Thanks!