Better robustness for stale mount points
Better robustness for stale mount points

For some stale mount points, (.exists file) will return
false, even they exists on the filesystem and cause
`Operation timed out` errors for command line tools.

It turns out that, for these, (.canRead file) returns true
(but false for other non-existent directories), and
provides a means to detect such situations.

Added a new method that incorporates `.canRead`.

Fixes #85

jobez/ambly,bsvingen/ambly,omcljs/ambly,omcljs/ambly,jobez/ambly,bsvingen/ambly,domesticmouse/ambly,domesticmouse/ambly