Type checkers for Python are great, but one of the things they do is they'll flag code as unreachable if there are branches which are checking the type of arguments, even if that branch exists in order to provide runtime type-checking of arguments for public APIs. `typing.assert_never` is designed for this case, but annoyingly it always raises an AssertionError, and does not allow customization of the message. This change adds a new method, `typelets.runtime.raise_invalid_type`, which can be used in the same way as `assert_never`, but defaults to raising a `ValueError` (and allows different exception types if desired), as well as taking in a message to include in the exception. This allows us to write APIs which perform runtime type checking on arguments without having the type checkers flag said checking as unreachable based on the type hints. Testing Done: Used this in some other code.