Improve error handling if webpack errors. (#1374)
Improve error handling if webpack errors. (#1374)

It turns out webpack can error in two ways. This adds a check for the second way
and also surfaces any warnings.
See: https://webpack.js.org/api/node/#error-handling.
FSMaxB/mathjs,josdejong/mathjs,josdejong/mathjs,FSMaxB/mathjs,FSMaxB/mathjs,josdejong/mathjs,FSMaxB/mathjs,FSMaxB/mathjs,josdejong/mathjs,josdejong/mathjs