Add missing @throws in stubs.

[hxv]

I've compared all stub files with Symfony's source files, and found some missing @throws tags:

• in Symfony\Contracts\Cache\CacheInterface::get() (source)
• in Symfony\Component\Console\Command\Command::getHelper() (source)
• in Symfony\Component\DependencyInjection\Extension\ExtensionInterface::load() (source)
• in Symfony\Component\Serializer\Encoder\EncoderInterface::encode() (source)
• in Symfony\Component\Serializer\Encoder\DecoderInterface::decode() (source)

Exceptions should now match.

[ondrejmirtes]

Thank you.