Fix audit creation with no name and no goal or audit_template

Currently, in that case it was failing because watcher tried to create a
name based on a goal automatically and the goal is not defined.

This patch is moving the check for goal specification in the audit
creation call earlier, and if there is not goal defined, it returns an
invalid call error.

This patch is also modifying the existing error for this case to check
the expected behavior.

Closes-Bug: #2110947

Change-Id: I6f3d73b035e8081e86ce82c205498432f0e0fc33
(cherry picked from commit bf6a28bd1e)
Signed-off-by: Alfredo Moralejo <amoralej@redhat.com>

releasenotes/notes/bug-2110947.yaml

@@ -0,0 +1,10 @@
+---
+fixes:
+  - |
+     Previously, when users attempted to create a new audit without providing
+     a name and a goal or an audit template, the API returned error 500 and an
+     incorrect error message was displayed.
+
+     Now, Watcher displays a helpful message and returns HTTP error 400.
+
+     For more info see: https://bugs.launchpad.net/watcher/+bug/2110947