QProcess doesn't always respect setWorkingDirectory

On both Mac OS X (Mavericks) and Linux (SLF 5.5), if you call QProcess::setWorkingDirectory() with a nonexistent directory name, it happily runs the process in the current working directory of the caller.

On Windows 7, it issues the error FailedToStart.

I believe the Windows behavior is correct.

As a courtesy to developers, it would be best to add a new error, InvalidWorkingDirectory, as we invariably search the program for reasons that it FailedtoStart, when this problem is in the calling code.