refined error response JSON schema

doc/api_endpoints.md

@@ -336,14 +336,12 @@ Explains why an irrelevant file is irrelevant.
 | Case | Code | Description | Response Body |
 | --- | --- | --- | --- | 
 | Ok | `200` | explanation(s) successfully created | JSON object with properties `explanations` and `further_explanations` (=boolean=) | 
-| Bad Request | `400` | item is not irrelevant and thus requesting an explanation is not possible | JSON object with properties `code` and `message` |
+| Bad Request | `400` | item is not irrelevant and thus requesting an explanation is not possible | an [Error response](<#json_error>) |
 
     - `explanations`: A list of explanations (see below).
     - `further_explanations`: `true` if more explanations are available
-    - `code`: the HTTP status code (400)
-    - `message`: an error message
 
-On success, the response body follows this [JSON schema](</doc/schema/response-explain-schema.json>). Otherwise, the response body follows the [error schema](</doc/schema/response-error-schema.json>).
+On success, the response body follows this [JSON schema](</doc/schema/response-explain-schema.json>).
 
 
 #### Explanations

doc/schema/response-error-schema.json

@@ -6,13 +6,23 @@
     "description": "The response body when the API returns an error.",
     "properties": {
         "code": {
+            "description": "The HTTP status code of this error.",
             "type": "integer",
-            "minimum": 400,
+            "minimum": 300,
             "maxiumum": 599
         },
         "message": {
+            "description": "A string describing the error.",
             "type": "string",
             "minimumLength": 1
+        },
+        "location": {
+          "description": "The HTTP location this error relates to (error-dependent).",
+          "type": "string"
+        },
+        "path": {
+          "description": "The URL path  (on this server) the error originates.",
+          "type": "string"
         }
     },
     "required": [