FIX: Replacing symlink/regular file by directory caused abort

Replacing a symlink/regular file in a repository by a directory might caused `cvmfs_server sync` to abort with `'<filename>' has an unsupported file type!`. This was partially fixed in [Pull Request #374](https://github.com/cvmfs/cvmfs/pull/374) but test case 531 failed again now.

When replacing a regular file or symlink by a directory and putting files in this newly created directory it will try to stat those files in the read-only branch. This leads to **ENOTDIR**. The code only checked for **ENOENT** though, and failed.
```
r/w:
   /foo/bar/regular_file/
   /foo/bar/regular_file/is_dir_now
rdonly:
   /foo/bar/regular_file   <-- ENOTDIR when asking for (.../is_dir_now)
```