Issue-949: Fixing inconsistencies in output schema

1. Changing "oneOf" to "anyOf", because basic, detailed and verbose have the same definition, oneOf would fail validation.
2. Adding "error" keyword and making either "error" or "errors" required in case "valid" is false.

Author: stan-sb

output/schema.json

@@ -3,7 +3,7 @@
   "$id": "https://json-schema.org/draft/2019-09/output/schema",
   "description": "A schema that validates the minimum requirements for validation output",
 
-  "oneOf": [
+  "anyOf": [
     { "$ref": "#/$defs/flag" },
     { "$ref": "#/$defs/basic" },
     { "$ref": "#/$defs/detailed" },
@@ -25,6 +25,9 @@
           "type": "string",
           "format": "uri-reference"
         },
+        "error": {
+          "type": "string"
+        },
         "errors": {
           "$ref": "#/$defs/outputUnitArray"
         },
@@ -41,7 +44,14 @@
             }
           },
           "then": {
-            "required": [ "errors" ]
+            "oneOf": [
+              {
+                "required": [ "error" ]
+              },
+              {
+                "required": [ "errors" ]
+              }
+            ]
           }
         },
         {