Fixed invalid panel path err in 3 out of 4 test cases

Wcm invalid path problem details:

Initial test case steps:
s1. open wcm
s2. change a panel to some folder
s3. exit wcm
s4. remove the folder
s5. open wcm again
Problem: Wcm shows empty panel on startup. It is not fatal though, the panel folder can still be changed to valid one e.g. via Alt-F1.

In the same situation Far silently switches to parent folder, recursively. The recursion fails, Far switches the panel to Far home folder.

Additional test cases:
c1. Removed folder is on local FS (original case)
c2. Removed folder is on SFTP virtual FS.
c3. Folder is on USB flash drive, s4 is to unplug the flash.
c4. Local folder on s4 is made unaccessible (chmod 000 on UX, or similar on Win).

The 2014-08-28 fix closes c1, c2, and c4. Wcm behaves exactly like Far.

c3, still fails. Wcm shows "Inaccessible" message, but does not escape to safe folder like Far does. Fixing c3 case  requires changing FS object from deep inside call stack. This requires more investigationon side efcts of the change.

The changes have been smoke-tested on Win, and Linux builds.