Throw InvalidArgumentException on invalid platform packages in config.

It's a user provided configuration value that's wrong so
it makes more sense.

src/Composer/Repository/PlatformRepository.php

@@ -49,7 +49,7 @@ class PlatformRepository extends ArrayRepository
                 parent::addPackage($package);
             }
             else {
-                throw new \UnexpectedValueException('Invalid platform package "'.$name);
+                throw new \InvalidArgumentException('Invalid platform package '.$name);
             }
         }