Kernel#require should transparently pass through raised Exceptions

[lopopolo]

The Rust implementation of Kernel#require wraps exceptions raised by eval in a RuntimeError. It should reraise transparently.

This test will fail once this is fixed:

https://github.com/lopopolo/ferrocarril/blob/329c544006b619b7b4fac9c96b6b5ec51a311a84/mruby/src/eval.rs#L356-L369