Fix derivation load assertion errors

When loading a derivation from a JSON, malformed input would trigger
cryptic "assertion failed" errors. Simply replacing calls to `operator []`
with calls to `.at()` was not enough, as this would cause json.execptions
to be printed verbatim.

Display nice error messages instead and give some indication where the
error happened.

*Before:*

```
$ echo 4 | nix derivation add
error: [json.exception.type_error.305] cannot use operator[] with a string argument with number

$ nix derivation show nixpkgs#hello | nix derivation add
Assertion failed: (it != m_value.object->end()), function operator[], file /nix/store/8h9pxgq1776ns6qi5arx08ifgnhmgl22-nlohmann_json-3.11.2/include/nlohmann/json.hpp, line 2135.

$ nix derivation show nixpkgs#hello | jq '.[] | .name = 5' | nix derivation add
error: [json.exception.type_error.302] type must be string, but is object

$ nix derivation show nixpkgs#hello | jq '.[] | .outputs = { out: "/nix/store/8j3f8j-hello" }' | nix derivation add
error: [json.exception.type_error.302] type must be object, but is string

```

*After:*

```
$ echo 4 | nix derivation add
error: Expected JSON of derivation to be of type 'object', but it is of type 'number'

$ nix derivation show nixpkgs#hello | nix derivation add
error: Expected JSON object to contain key 'name' but it doesn't

$ nix derivation show nixpkgs#hello | jq '.[] | .name = 5' | nix derivation add
error: Expected JSON value to be of type 'string' but it is of type 'number'

$ nix derivation show nixpkgs#hello | jq '.[] | .outputs = { out: "/nix/store/8j3f8j-hello" }' | nix derivation add
error:
       … while reading key 'outputs'

       error: Expected JSON value to be of type 'object' but it is of type 'string'
```

1 files changed, 2 insertions, 0 deletions

diff --git a/doc/manual/src/release-notes/rl-next.md b/doc/manual/src/release-notes/rl-next.md
index df30ce83d..526b64fde 100644
--- a/doc/manual/src/release-notes/rl-next.md
+++ b/doc/manual/src/release-notes/rl-next.md
@@ -8,3 +8,5 @@
   These functions are useful for converting between flake references encoded as attribute sets and URLs.
 
 - [`builtins.toJSON`](@docroot@/language/builtins.md#builtins-parseFlakeRef) now prints [--show-trace](@docroot@/command-ref/conf-file.html#conf-show-trace) items for the path in which it finds an evaluation error.
+
+- Error messages regarding malformed input to [`derivation add`](@docroot@/command-ref/new-cli/nix3-derivation-add.md) are now clearer and more detailed.