When a directory cannot be walk-ed because of a permissions error
- or any error for that matter -, ncdu mode keeps track of the error
and highlights directories that could not be read.
Previously, the error would cause ncdu to abort.
Now, directories with unreadable sub-directories are displayed in yellow and
a message warns that the total may be underestimated.
Unreadable directories themselves are displayed in red along with the error message
f7fe1d766b