Throw exception on patch failure if env var is set

src/Patches.php

@@ -165,7 +165,7 @@ public function gatherPatches(PackageEvent $event) {
     // won't hurt anything, so we'll just stash it there.
     $this->patches['_patchesGathered'] = TRUE;
   }
-  
+
   /**
    * Get the patches from root composer or external file
    * @return Patches
@@ -263,6 +263,9 @@ public function postInstall(PackageEvent $event) {
       }
       catch (\Exception $e) {
         $this->io->write('   <error>Could not apply patch! Skipping.</error>');
+        if (getenv('COMPOSER_EXIT_ON_PATCH_FAILURE')) {
+          throw new \Exception("Cannot apply patch $description ($url)!");
+        }
       }
     }
     $localPackage->setExtra($extra);