add 400-error info. to PUT request in YAML file.

app/swagger.yml

@@ -79,6 +79,11 @@ paths:
         201:
           description: >-
             The reading was successfully added to the database.
+        400:
+          description: >-
+            The data sent to the sever was invalid or incorrectly
+            formatted. The most likely reasons are invalid light-meter
+            Id. specified and incorrect datetime format used.
 
   /readings/latest/{light_meter}:
     get:
@@ -265,6 +270,11 @@ paths:
         201:
           description: >-
             The status update was successfully added to the database.
+        400:
+          description: >-
+            The data sent to the sever was invalid or incorrectly
+            formatted. The most likely reasons are invalid device Id.
+            specified and incorrect datetime format used.
 
   /status/latest/{device}:
     get: