Fix nonsense error messages in shutil.py

Attempting to copy a file onto a symlink to itself previously gave the error
```
shutil.Error: ['<', 'D', 'i', 'r', 'E', 'n', 't', 'r', 'y', ' ', "'", 's', 't', 'y', 'l', 'e', '.', 'c', 's', 's', "'", '>', ' ', 'a', 'n', 'd', ' ', "'", 'b', 'u', 'i', 'l', 'd', '/', 'h', 't', 'm', 'l', '/', 's', 't', 'y', 'l', 'e', '.', 'c', 's', 's', "'", ' ', 'a', 'r', 'e', ' ', 't', 'h', 'e', ' ', 's', 'a', 'm', 'e', ' ', 'f', 'i', 'l', 'e']
```

Author: eric-wieser

Lib/shutil.py

@@ -500,7 +500,11 @@ def _copytree(entries, src, dst, symlinks, ignore, copy_function,
         # catch the Error from the recursive copytree so that we can
         # continue with other files
         except Error as err:
-            errors.extend(err.args[0])
+            if isinstance(err.args[0], list):
+                # this is a recursive error
+                errors.extend(err.args[0])
+            else:
+                errors.append((srcname, dstname, str(err)))
         except OSError as why:
             errors.append((srcname, dstname, str(why)))
     try: