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, 22 insertions, 0 deletions

diff --git a/src/libutil/json-utils.hh b/src/libutil/json-utils.hh
index 5e63c1af4..77c63595c 100644
--- a/src/libutil/json-utils.hh
+++ b/src/libutil/json-utils.hh
@@ -11,6 +11,28 @@ const nlohmann::json * get(const nlohmann::json & map, const std::string & key);
 nlohmann::json * get(nlohmann::json & map, const std::string & key);
 
 /**
+ * Get the value of a json object at a key safely, failing
+ * with a Nix Error if the key does not exist.
+ *
+ * Use instead of nlohmann::json::at() to avoid ugly exceptions.
+ *
+ * _Does not check whether `map` is an object_, use `ensureType` for that.
+ */
+const nlohmann::json & valueAt(
+    const nlohmann::json & map,
+    const std::string & key);
+
+/**
+ * Ensure the type of a json object is what you expect, failing
+ * with a Nix Error if it isn't.
+ *
+ * Use before type conversions and element access to avoid ugly exceptions.
+ */
+const nlohmann::json & ensureType(
+    const nlohmann::json & value,
+    nlohmann::json::value_type expectedType);
+
+/**
  * For `adl_serializer<std::optional<T>>` below, we need to track what
  * types are not already using `null`. Only for them can we use `null`
  * to represent `std::nullopt`.