a failure inside MakeChunkWritable should call abort/fassert instead of assert

At the end of makeChunkWritable (in mongommf.cpp in 2.0.x and currently in mmap_win.cpp in 2.1.x), if the call to VirtualProtect does not return "ok", abort/fassert should be called instead of assert/verify. If this call fails, there may be data corruption, so the server should shut down to allow recovery via the journal.