Add error.* fields to spec (#35)

Author: felixbarny

spec/spec.json

@@ -104,6 +104,24 @@
             "required": false,
             "url": "https://www.elastic.co/guide/en/ecs/current/ecs-log.html",
             "comment": "Should be opt-in as it requires the logging library to capture a stack trace for each log event."
+        },
+        "error.type": {
+            "type": "string",
+            "required": false,
+            "url": "https://www.elastic.co/guide/en/ecs/current/ecs-error.html",
+            "comment": "The exception type or class, such as `java.lang.IllegalArgumentException`."
+        },
+        "error.message": {
+            "type": "string",
+            "required": false,
+            "url": "https://www.elastic.co/guide/en/ecs/current/ecs-error.html",
+            "comment": "The message of the exception."
+        },
+        "error.stack_trace": {
+            "type": "string",
+            "required": false,
+            "url": "https://www.elastic.co/guide/en/ecs/current/ecs-error.html",
+            "comment": "The stack trace of the exception as plain text."
         }
     }
 }