Check for 200 response code.

Without this check, missing sourcemaps cause the package to continue as
if the 404 message is the sourcemap, leading to crashes when responses
like "Not Found" cannot be parsed as source maps.

source-map-support.js

@@ -29,7 +29,10 @@ function retrieveFile(path) {
       var xhr = new XMLHttpRequest();
       xhr.open('GET', path, false);
       xhr.send(null);
-      var contents = xhr.readyState === 4 ? xhr.responseText : null;
+      var contents = null
+      if (xhr.readyState === 4 && xhr.status === 200) {
+        contents = xhr.responseText
+      }
     }
 
     // Otherwise, use the filesystem