No error message if --html-dir is not specified correctly, or no files found

[cyclops1982]

The instructions clearly state to run ./build/trifecta and this of course works.
However, if you run it from another directory (for example, using ./trifecta from the build directory), then the pages render empty. This is because the arg html-dir points to the relative path. Of course, you can fix this by adding --html-dir

The software could however be friendlier in providing an error message. If the html-dir is wrongly provided, or trifecta is run from another directory, then an empty page is rendered. This is confusing and /should/ actually be a 404 error message.