separate errors for missing and dangling basedir links

Also the os.listdir call is put inside a try/except to prevent
crashing the app.
rename (COLLECTION|ENTITY|FILE)_FIELDS var to FIELDS

Use the same variable in each document module to make it easier
to write functions that operate on multiple modules.
mv sample FIELDS definition to ddrlocal.models.collection
add MODEL field to ddrlocal.models.(collection,entity,files)

Just ask the module for the model instead of trying to extract
it from the path or something equally silly.
consolidate redundant model display and form prep/post code