app_server: Fix potential infinite loop

Return `B_ENTRY_NOT_FOUND` instead of -1 when `/dev/graphics`
is not found.

Otherwise, `app_server` would run into an infinite loop
while waiting `fCardFD` to equal `B_ENTRY_NOT_FOUND` in some
specific environments such as a `chroot` where `/dev` is missing.

Change-Id: Ice23a82f58811f1258c58826c2488ae5c5c29cee
Reviewed-on: https://review.haiku-os.org/c/haiku/+/6376
Reviewed-by: X512 <[email protected]>
Tested-by: Commit checker robot <[email protected]>
Reviewed-by: Jérôme Duval <[email protected]>

src/servers/app/drawing/interface/local/AccelerantHWInterface.cpp

@@ -239,7 +239,7 @@ AccelerantHWInterface::_OpenGraphicsDevice(int deviceNumber)
 	if (!use_fail_safe_video_mode()) {
 		DIR *directory = opendir("/dev/graphics");
 		if (!directory)
-			return -1;
+			return B_ENTRY_NOT_FOUND;
 
 		struct dirent *entry;
 		char path[PATH_MAX];

src/servers/app/drawing/interface/virtual/DWindowHWInterface.cpp

@@ -407,7 +407,7 @@ DWindowHWInterface::_OpenGraphicsDevice(int deviceNumber)
 {
 	DIR *directory = opendir("/dev/graphics");
 	if (!directory)
-		return -1;
+		return B_ENTRY_NOT_FOUND;
 
 	// TODO: We do not need to avoid the "vesa" or "framebuffer" drivers this way
 	// once they been ported to the new driver architecture - the special case here