Improve error reporting and standardise assertions.

[luk707]

Currently errors thrown from Funxion are just strings. It would make sense to create more specific error classes that inherit Error and provide more useful information from the user. This will also provider an API for users to make assertions on when validating user provided expressions.

AC

• Outline required error classes
• Implement error classes
• Document errors and provide validation assertion examples
• Add tests to ensure errors are raised as expected and stable to rely upon