[Makefile] Understandable error messages from uglifyjs

When running make, I used to get the following error message on a parse error, regardless of what file actually contained the parse error:
WARN: ERROR: Unexpected token: eof (undefined) [-:2019,8]

With this, change, the error message is actually understandable, ie:
WARN: ERROR: Unexpected token: name (asdfkl) [nv.d3.js:6349,4]