Detect invalid tile files

[CrazyFluffyPony]

Thank you for this nice repository!

Maybe you could add a check for the tiles {z}/{x}/{y}.png being valid png files. If misconfigured (i had maxzoom set to 12 but only had 10) and the webserver returns e.g. an html error page, there is a weird 'InvalidStateError' from MapLibre GL JS at map.addImage()

Totally a user error, but might be worth to add. Thank you!