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, 24 insertions, 0 deletions

diff --git a/src/libutil/json-utils.cc b/src/libutil/json-utils.cc
index d7220e71d..61cef743d 100644
--- a/src/libutil/json-utils.cc
+++ b/src/libutil/json-utils.cc
@@ -1,4 +1,5 @@
 #include "json-utils.hh"
+#include "error.hh"
 
 namespace nix {
 
@@ -16,4 +17,27 @@ nlohmann::json * get(nlohmann::json & map, const std::string & key)
     return &*i;
 }
 
+const nlohmann::json & valueAt(
+    const nlohmann::json & map,
+    const std::string & key)
+{
+    if (!map.contains(key))
+        throw Error("Expected JSON object to contain key '%s' but it doesn't", key);
+
+    return map[key];
+}
+
+const nlohmann::json & ensureType(
+    const nlohmann::json & value,
+    nlohmann::json::value_type expectedType
+    )
+{
+    if (value.type() != expectedType)
+        throw Error(
+            "Expected JSON value to be of type '%s' but it is of type '%s'",
+            nlohmann::json(expectedType).type_name(),
+            value.type_name());
+
+    return value;
+}
 }