Cavern and Aven should throw a more-helpful error when directory is not writeable

[Eric C. Landgraf]

I spent more time than I'd like to admit trying to figure out why a walls dataset with no errors was erroring out with error: Couldn’t open file “<FILENAME>.3d” What was not clear is that it could not open it for WRITING, rather than for reading.

Also in unexpected behaviours: if the 3d file does exist but is not writeable, aven will successfully open the *old* file after failing to write one.