FieldMode: Add NotAllowed error

The FieldMode property is a one-way property, add support to return a
NotAllowed error when it's requested to be cleared.

Tested: Verify an error is created when a value of 0 was requested via
        curl -b cjar -k -H "X-Auth-Token: $sha" -X PUT -d '{"data":0}' https://$bmc/xyz/openbmc_project/software/attr/FieldModeEnabled

Change-Id: I7121026f3dd63eef5a14e4b932ae7649e3c273e6
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/xyz/openbmc_project/Control/FieldMode.interface.yaml b/xyz/openbmc_project/Control/FieldMode.interface.yaml
index 4cbe6c8..639fa43 100644
--- a/xyz/openbmc_project/Control/FieldMode.interface.yaml
+++ b/xyz/openbmc_project/Control/FieldMode.interface.yaml
@@ -7,5 +7,9 @@
       type: boolean
       description: >
           Keeps track of whether the BMC is in field mode. Enabling field mode
-          is intended to be a one-way operation.
+          is intended to be a one-way operation. Therefore the application
+          implementing this interface could return a NotAllowed error when the
+          user attempts to set the property value to false.
       default: false
+      errors:
+        - xyz.openbmc_project.Common.Error.NotAllowed