Handle directory conflicts with html output.

When calling html_report() if the output path already exists
instead of rewriting the files in the directory html_report
throws OSError: [Errno 17] File exists which leads to a 500
return to the api call. This commit avoids this issue by
checking if the directory exists beforehand and returning a
400 with an error message instead.

Change-Id: I061534c9ce9977692a7fc98187c4797b2ed60ab2

nova/api/openstack/compute/contrib/coverage_ext.py

@@ -203,6 +203,9 @@ class CoverageController(object):
             if xml:
                 self.coverInst.xml_report(outfile=path)
             elif html:
+                if os.path.isdir(path):
+                    msg = _("Directory conflict: %s already exists")
+                    raise exc.HTTPBadRequest(explanation=msg)
                 self.coverInst.html_report(directory=path)
             else:
                 output = open(path, 'w')