Add yaml test

Author: thecoop

plugins/examples/rescore/src/yamlRestTest/resources/rest-api-spec/test/example-rescore/30_factor_field.yml

@@ -48,6 +48,21 @@ setup:
   - match: { hits.hits.1._score: 20 }
   - match: { hits.hits.2._score: 10 }
 
+---
+"referencing a missing field returns bad request":
+  - do:
+      catch: bad_request
+      search:
+        index: test
+        body:
+          rescore:
+            example:
+              factor: 1
+              factor_field: missing
+  - match: { status: 400 }
+  - match: { error.root_cause.0.type: "illegal_argument_exception" }
+  - match: { error.root_cause.0.reason: "Missing value for field [missing]" }
+
 ---
 "sorted based on a numeric field and rescored based on a factor field using a window size":
   - do: