Fix framework coverage commenting action

This commit handles the case when the current run finds no coverage change and the previous run is identified,
but it doesn't have the required artifacts.

misc/scripts/library-coverage/comment-pr.py

@@ -61,12 +61,15 @@ def comment_pr(repo, run_id):
 
     # Try storing diff for previous run:
     prev_run_id = 0
+    prev_diff_exists = False
     try:
         prev_run_id = get_previous_run_id(repo, run_id, pr_number)
         prev_diff_folder = "prev_diff"
         utils.download_artifact(repo, comparison_artifact_name,
                                 prev_diff_folder, prev_run_id)
 
+        prev_diff_exists = True
+
         if filecmp.cmp(f"{current_diff_folder}/{comparison_artifact_file_name}", f"{prev_diff_folder}/{comparison_artifact_file_name}", shallow=False):
             print(
                 f"Previous run {prev_run_id} resulted in the same diff, so not commenting again.")
@@ -74,7 +77,7 @@ def comment_pr(repo, run_id):
         else:
             print(f"Diff of previous run {prev_run_id} differs, commenting.")
     except Exception:
-        # this is not mecessarily a failure, it can also mean that there was no previous run yet.
+        # this is not necessarily a failure, it can also mean that there was no previous run yet.
         print("Couldn't generate diff for previous run:", sys.exc_info()[1])
 
     comment = get_comment_text(
@@ -82,9 +85,17 @@ def comment_pr(repo, run_id):
 
     if comment == None:
         if prev_run_id == 0:
-            print("Nothing to comment.")
+            print(
+                "Nothing to comment. There's no previous run, and there's no coverage change.")
             return
+
         print("Previous run found, and current run removes coverage change.")
+
+        if not prev_diff_exists:
+            print(
+                "Couldn't get the comparison artifact from previous run. Not commenting.")
+            return
+
         comment = comment_first_line + \
             "A recent commit removed the previously reported differences."
     post_comment(comment, repo, pr_number)