Do not error if source map path does not exist

If a file references a source map that does not exist, Plumber will error when we try to read the non-existent file. I think the right thing to do here is just silently fail? Perhaps a warning in the console so consumers can fix the problem and/or go and bug library maintainers to fix the problem?

In my case, it's [underscore.js](https://www.npmjs.org/package/underscore)’s `underscore-min.js` that contains a mapping to a non-existent file.