"Writable" filesystem though creation of 'sites/default/files' failed

When autocreating the filesystem 'sites/default/files' fails on install, the following message is displayed:

The file system is said to be "Writable" even though the directory doesn't exist.
While few users will be openly confused about this message, it is slightly irritating. They also get the wrong conception that the "writable file system" must be something else than the 'sites/default/files' directory.
Reason is that in system.install, the successful test of the existing os_temp directory overwrites the 'value' string.
If we do continue at all after an error (not so sure if that's good, either), then a successful subtask should never get to overwrite the error message of a previous failed subtask.