Handle non-UTF-8 filenames in cache directory when expiring

src/calibre/gui2/viewer/convert_book.py

@@ -54,8 +54,11 @@ def safe_makedirs(path):
 def robust_rmtree(x):
     retries = 2 if iswindows else 1  # retry on windows to get around the idiotic mandatory file locking
     for i in range(retries):
+        try:
             try:
                 rmtree(x)
+            except UnicodeDecodeError:
+                rmtree(as_bytes(x))
             return True
         except EnvironmentError:
             time.sleep(0.1)