Fail more gracefully when a rego rule references a custom rego function that isn't supported

Now that we're introducing custom rego functions, we should consider how to avoid hard failures when the function isn't supported in the available verison of ec-cli.

Ideas:

• Interrogate ec to provide a list of available custom functions and handle the case where the function isn't there.

Acceptance Criteria:

• ec produces a helpful error message when it tries to compile rego that includes custom rego functions it doesn't know about (but may be defined in a newer version of ec)
• The error message suggests potential solutions.