Improve error message for invalid JSON values

[Mrtenz]

Currently we use assertStruct in the JsonStruct, to ensure that the value is JSON-serialisable before coercing it. assertStruct returns a generic AssertionError, and Superstruct doesn't have any information about where the error was thrown (such as the path). Given the following struct for example:

const ExampleStruct = object({
  value: JsonStruct,
});

An invalid value would result in an AssertionError with the following message:

Assertion failed: Expected a value of type JSON, but received: undefined.

After this change, a StructError is thrown instead, with the following message:

At path: value -- Expected a value of type JSON, but received: undefined.

This makes it more clear that the error happens at value, and it also makes more sense to throw a StructError in this case.