There should be a warning when opening/embossing 8-dot

[joeha480]

Previously, there was a warning when opening or trying to emboss a file with 8-dot patterns. This has been lost when moving to JavaFX. Restore this, so that paper isn't wasted on files that cannot be embossed.

Note that even if #5 is fixed, there could still be situations where a warning could be necessary, for example when a specific embosser cannot support 8-dot.