Add more info to Could not produce font face declaration warning?

[peteruithoven]

Currently we're getting the following error:

WARN  Could not produce font face declaration from local for font family Faro Variable.

We have no idea why.

After adding a src to the family config, a tip from #779, it does work. So the problem was it couldn't find the font file?
It would be really useful to include the expected file name name / path. I realize there is some flexibility, like the supported extensions, so this might require some exploration. One way:

WARN  Could not produce font face declaration from local for font family Faro Variable. Could not find "faro-variable.[woff2|woff|ttf|otf|eot]".