Check that the nwb-schema is available

[oruebel]

When cloning PyNWB from git, users sometimes forget to include the --recurse-submodules parameter on check-out. This in turn leads to a KeyError: "'core' not a namespace" in HDMF because the core namespace for NWB is missing. It would be nice to catch this problem and issue a more informative message to indicate that the schema files for the NWB core namespace are missing so that the error is raised directly from PyNWB, rather than delayed from HDMF.

Checklist

• [X] Have you ensured the feature or change was not already reported?
• [X] Have you included a brief and descriptive title?
• [X] Have you included a clear description of the problem you are trying to solve?
• [X] Have you included a minimal code snippet that reproduces the issue you are encountering?
• [X] Have you checked our Contributing document?