property: Add errors support

Add errors support for properties.

Closes openbmc/openbmc#1641

Tested: Verified that when an interface threw a defined property
error, the generated code caught it and returned an error to
the REST call.

Change-Id: I8cb8c77995026f12e373291822e4e0a623a84d81
Signed-off-by: Adriana Kobylak <anoo@us.ibm.com>
diff --git a/example/net/poettering/Calculator.interface.yaml b/example/net/poettering/Calculator.interface.yaml
index d289c21..af4dd1b 100644
--- a/example/net/poettering/Calculator.interface.yaml
+++ b/example/net/poettering/Calculator.interface.yaml
@@ -55,6 +55,12 @@
       default: Success
       description: >
         The current state of the Calculator.
+    - name: Owner
+      type: string
+      description: >
+        The name of the owner of the Calculator.
+      errors:
+        - self.Error.PermissionDenied
 signals:
     - name: Cleared
       description: >