Catch `NullPointerException` and raise a more informative error

[bplommer]

NullPointerExceptions is raised when a codec depends on another one defined as a val further down in the same object. We should catch these exceptions and raise an error that suggests a fix (replacing val with lazy val).

[mjftw]

I'm currently fixing an error we're having around this and a more informative error message would be super helpful!

[fredrikljung93]

I'm new to Vulcan and wasted many hours trying to figure this out!