Better checking and error messages

[briandoconnor]

generate_metadata.py (which should probably be renamed at some point) will need to have error checking and better error messages to catch common problems that will come up as users use the tool to upload to use. Including but not limited to:

• incorrect file paths
• incorrect analysis types or other CV/ontology terms that we use (e.g. typos)
• missing columns, required fields
• inconsistent donor/specimen/samples (in one line a specimen is for one donor, another line it's for a different donor which cannot happen)
• etc