#102 Update CurlMulti code with Prefetcher.php from prestissimo

[Schnitzel]

this updates the code on how to use CurlMulti to the same code that is also running in https://github.com/hirak/prestissimo/blob/master/src/Prefetcher.php#L20

solves #102

[webflo]

I had changed the error handling in #88 so the code can throw an exception. Without the change only a log msg to std error.

[andypost]

+1 to fix - it works great!

     "patches": {
+      "drupal-composer/drupal-scaffold": {
+        "Parallel downloads": "https://github.com/drupal-composer/drupal-scaffold/pull/103.patch"
+      },

[webflo]

@Schnitzel Could you take a look at the error handling in v2.6.0 and report if the issue still applies?