Add Dutch language files

[fritsbits]

Return error messages in Dutch.
Now we only have them in English here: https://github.com/Prisma/api/tree/46ad4294f4a339525a9e89f8da11d99a7a5db4ae/resources/lang

These strings are already translated. Let's add them.
https://github.com/caouecs/Laravel-lang/tree/master/src/nl