Handle error on write-protected directories better

[Teteros]

SAGE games like Generals can default their installation to C:\Program Files or C:\Program Files (x86) on x64 systems which you need UAC/Administrator privilidges to write for.

easage will fail if it does not have a "Run as Administrator" compatibility flag set, or is not elevated in other ways with I/O errors such as:

on easage pack

and

on easage unpack

We should let the user know why reading/writing is failing and possibly offer to elevate permissions (UAC Yes/No prompt) if easage detects it has no read/write permissions in output folder.