Fix error handling in PrestissimoFileFetcher

[webflo]

Fixes #73

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I looked at the prestissimo code again, a this is the proper fix for this.

[FlorentTorregrosa]

Error got catched.

root@26130eede5ce:/project# composer drupal:scaffold                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
  [Exception]                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                         
  Failed to download https://fake.com/pantheon-systems/drops-8/8.5.4/sites/example.sites.php, https://fake.com/pantheon-systems/drops-8/8.5.4/sites/default/default.settings.php, https://fake.com/pantheon-systems/drops-8/8.5.4/sites/default/default.services.yml, https://fake.com/pantheon-systems/drops-8/8.5.4/robots.txt, https://fake.com/pantheon-systems/drops-8/8.5.4/index.php, https://fake.com/pantheon-systems/drops-8/8.5.4/.htaccess, https://fake.com/pantheon-systems/drops-8/8.5.4/.gitattributes, https://fake.com/pantheon-systems/drops-8/8.5.4/.eslintrc.json, https://fake.com/pantheon-systems/drops-8/8.5.4/.eslintignore, https://fake.com/pantheon-systems/drops-8/8.5.4/.editorconfig  
                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                      
drupal:scaffold