Fix handling of dangling symlink on manifest generation

There may be broken symlinks within the log directories, those fail with
an error when os.stat() is executed on them. Let's just skip those.

Change-Id: I3e6982c53a08f57ac0e592b8a0041bbb39812d1f

roles/generate-zuul-manifest/library/generate_manifest.py

@@ -67,6 +67,8 @@ def walk(root, original_root=None):
         mime_guess, encoding = mimetypes.guess_type(path)
         if not mime_guess:
             mime_guess = 'text/plain'
+        # This may fail e.g. for dangling symlinks, just ignore those
+        try:
             st = os.stat(path)
             last_modified = st[stat.ST_MTIME]
             size = st[stat.ST_SIZE]
@@ -75,6 +77,9 @@ def walk(root, original_root=None):
                              encoding=encoding,
                              last_modified=last_modified,
                              size=size))
+        except FileNotFoundError:
+            continue
+
     return data