CreateBranchIT#assertCreateFails: Restructure to avoid ErrorProne internal error

The current structure of the code causes an IllegalStateException
in ErrorProne [1]. Restructure the code to avoid it.

[1] https://github.com/google/error-prone/issues/1072

Change-Id: I17c96eca6c8dc8b0e234d504e32dda49d9f4720a
diff --git a/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/rest/project/CreateBranchIT.java b/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/rest/project/CreateBranchIT.java
index 766089b..1b9a34a 100644
--- a/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/rest/project/CreateBranchIT.java
+++ b/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/rest/project/CreateBranchIT.java
@@ -98,10 +98,10 @@
 
   private void assertCreateFails(Class<? extends RestApiException> errType, String errMsg)
       throws Exception {
-    exception.expect(errType);
     if (errMsg != null) {
       exception.expectMessage(errMsg);
     }
+    exception.expect(errType);
     branch().create(new BranchInput());
   }