[luatex] Prevent stop when tiff file loads

[Patrick Gundlach]

Hello

> In practice, identifying all image files formats (tiff, png, pdf...) *should* work with the first 4 bytes of the file (they were designed this way). 

I think I'll go that way, but I still wish (feature request) that LuaTeX does not fail with a fatal error if an unknown file format is encountered, instead it would be nice to have something like "return nil, errormessage".

Thanks

   Patrick