Avoid redundant error message

Summary:
Right now, running tests can give:
```
Expected an issue with code 5005, but no issue was found.
Expected 1 issues with code 5005, but found 0 issues.
```

This can be confusing since this is referring to the same problem. Let's fix it.

Reviewed By: tianhan0

Differential Revision: D64470602

fbshipit-source-id: 5c8615305053c6f8e87439575cea777078caba11

tools/pysa_integration_tests/runner_lib.py

@@ -668,9 +668,9 @@ def compare_issues_to_test_annotations(
             else:
                 pass  # pass
 
-    if len(issues) != number_expected_issues:
+    if len(issues) != number_expected_issues and len(test_failures) == 0:
         test_failures.append(
-            f"Expected {number_expected_issues} issues with code {code}, but found {len(issues)} issues"
+            f"Expected {number_expected_issues} issues with code {code}, but found {len(issues)} issues."
         )
 
     if len(test_failures) > 0: