Fix exceptions in all services

Quick fix: Now, every method throws a NotFoundException in all methods in the services. What we really want is to throw a 404 only when an object is really missing. In other situations (e.g. when connecting to db fails) getting not found could be misleading, therefore we should throw the corresponding exception or we wouldn't even need exception handling.

Start by reviewing all service methods that currently throw NotFoundException and trace how missing objects differ from database connection failures. Done means missing objects produce 404 responses while connection or other failures use the corresponding exception instead of being reported as not found.

Written by the indexing model from the issue text.