fontspec throwing error with lualatex - broken?

You might be missing a required package, try:

sudo apt-get install texlive-luatex

Then, try the update:

luaotfload-tool --update

I have just faced the same problem and it turns out that the solution was to install the lmodern package. If I understand correctly this also installs the fonts-lmodern package which contains the missing font file lmroman10-regular that lualatex was complaining about. (lmodern seems to contain font files for pdfTeX)

The conclusion is that this is perhaps not a TeX problem but rather a package dependency problem in the Linux Mint distribution (I am on Linux Mint Debian Edition). Still it seems a bit weird that lualatex requires a font that is not even used in the document.