Io: Consistently use error codes with OpenFile().

This gets rid of OpenWithError(), and just always returns a negative value
on error for OpenFile().  Also fixed the sequence rollover, which could've
returned 0.

0 should be considered a valid handle ideally, but left it never returning
0 to simplify cleanup in some areas.

UI/GameInfoCache.cpp

@@ -301,7 +301,7 @@ static bool ReadFileToString(IFileSystem *fs, const char *filename, std::string
 	}
 
 	int handle = fs->OpenFile(filename, FILEACCESS_READ);
-	if (!handle) {
+	if (handle < 0) {
 		return false;
 	}