dtb/meta-data fails with FileNotFoundException if there is no ncc.html

[egli]

mdr2.encode> (dtb/meta-data (path/recorded-path (prod/find 16666)))
FileNotFoundException /home/eglic/tmp/mdr2/production/recorded/16666/ncc.html (No such file or directory)  java.io.FileInputStream.open (FileInputStream.java:-2)

[egli]

This shouldn't really be a problem as such a dtb is most likely not valid. OK, granted if it was a DAISY3 DTB it would probably pass the validator. And then it would have an ncx file instead of a ncc.html. But then the whole depth function would break anyway. Let's cross this river when we get to it, i.e. when/if we ever produce DAISY3.