Add PreconditionFailed exception for HTTP 412 errors

This will be an expected error code from Neutron API tests
when testing the compare-and-swap update capabilities being
added in I7d97d6044378eb59cb2c7bdc788dc6c174783299.

Otherwise the tests need to catch UnexpectedResponseCode and
extract the error number from the body, which looks ugly.

Change-Id: Ic2576438228ea1e15de01779735f7c3ca048c4e5
diff --git a/tempest/lib/common/rest_client.py b/tempest/lib/common/rest_client.py
index d0e21ff..f5bff20 100644
--- a/tempest/lib/common/rest_client.py
+++ b/tempest/lib/common/rest_client.py
@@ -617,6 +617,7 @@
         :raises BadRequest: If a 400 response code is received
         :raises Gone: If a 410 response code is received
         :raises Conflict: If a 409 response code is received
+        :raises PreconditionFailed: If a 412 response code is received
         :raises OverLimit: If a 413 response code is received and over_limit is
                           not in the response body
         :raises RateLimitExceeded: If a 413 response code is received and
@@ -775,6 +776,11 @@
                 resp_body = self._parse_resp(resp_body)
             raise exceptions.Conflict(resp_body, resp=resp)
 
+        if resp.status == 412:
+            if parse_resp:
+                resp_body = self._parse_resp(resp_body)
+            raise exceptions.PreconditionFailed(resp_body, resp=resp)
+
         if resp.status == 413:
             if parse_resp:
                 resp_body = self._parse_resp(resp_body)