Throws when source map is not found

[LarsDenBakker]

When a file points to a source map that doesn't exist it currently throws an error:

Error: An error occurred while trying to read the map file at <file path>
Error: ENOENT: no such file or directory, open <file path>

I'm trying to parse source maps delivered from the browser, so paths might not always match up perfectly. Would it make sense to have an option to ignore the error and continue without source maps?

[bcoe]

@LarsDenBakke it seems reasonable to me that our default behavior would be to console.warn what do you think? an error seems a little heavy handed.