Silent crash if there are broken symlinks in the project folder

[reitzig]

If there are broken symlinks among the files copied to the tmp folder, ltx2any silently crashes.

First of all, make the error loud like all others, and have it point to _.err (which is written).

Second, copy2tmp "copies" symlinks be creating new ones to the target. Apparently, File.symlink does not like it if it's told to symlink to a broken symlink.