Add helpful error message for missing module docker image

[chasemc]

Add helpful error message if module's docker isn't available (e.g. "Try to running make modules-images then running nextflow again")

To prevent just seeing:

Unable to find image 'jasonkwan/autometa-nf-modules-mock_data_reporter:latest' locally

[evanroyrees]

This issue should be resolved once #216 is merged. These images will be hosted after Jason adds his docker credentials.

[chasemc]

That makes sense