Add option to prevent throw error when not found matching file

[dungtran5]

Hi author,

Recently I used this middleware, I had a issue that when a request for a sourcemap file not found in server (because I deleted it manually)

As I read in nodejs document here https://nodejs.org/api/fs.html#fs_fs_statsync_path_options We could avoid this throw by skip it (as my case)

So I hope this PR would add a option for anyone have the same issue with me.

[dungtran5]

I'll add some test for this

[dungtran5]

Close as I ended up using other libraries. Thanks author