Fix pecan error message not available in body for Trigger.post

When a trigger_id resource is not found, the API returns a 202 status
code, the error message could not be added in the body of the request
because pecan.response.text was used instead of pecan.response.body.

Change-Id: I8b03210b5a2f2b5c0ea24bfc8149cca122dffeea
Closes-Bug: #1324940
Merge "Fix pecan error message not available in body for Trigger.post"