Displayed paths should be canonized

[crowdagger]

Sometimes, depending on where you run Crowbook from, the printed paths for the documents that can be generated can be unnecessarily verbose, e.g. some_dir/../output/foo.pdf instead of output/foo.pdf. A previous attempt to fix this was done using std::fs::canonicalize, but it caused issues as this function will return an error if the file doesn't already exist.