MCGitHubRepository class>>resetCacheDirectoryIfInvalid not pessimistic enough

[dalehenrich]

If the directory exists but the files within the directory are not readable by the user running the image, the system breaks down ...

I guess we could try reading a file in the directory, but even then there are no guarantees ... I don't like the idea of handling the error by clearing the cache and retrying ... since there may be another problem altogether involved ... perhaps we could determine if we have write access on the file ... that would probably be sufficient ... but we'll have to expand the file utility api to do a permissions check and get this working on all supported platforms