Protect folders Ships/[VAB|SPH] from deletion.

Throw a DirectoryNotFoundKraken if trying to write a file and part of its
path is missing (usually Ships and/or VAB/SPH, see #3 and #5).
Should be catched in cmdline and gui.
Merge pull request #18 from Ormira/Issue#3

Protect folders Ships/[VAB|SPH] from deletion.