Improved error messages for narratives

Current Behavior

Dropping narratives (with or without associated datasets) onto auspice.us doesn't work - error notification "auspice.us couldn't load any of the dropped files!". Example console message:

Read flu_seasonal_h3n2_ha_2y.json as a main dataset JSON file
Read test_simultaneous-tree-updates.md as a narrative.
Linking narrative datasets to dropped JSON datasets.
Narrative slide URL flu/seasonal/h3n2/ha/3y expected flu_seasonal_h3n2_ha_3y.json but this wasn't found.

Behavior exists at auspice.us 0.12.0 (Auspice 2.46.0) and is unchanged in #59, so I'm going to merge that now.

I thought for auspice.us, you must include a dataset JSON with the narratives?

auspice.us/CHANGELOG.md

Line 6 in 754ac3b

    
           * A narrative (markdown) can be loaded, as long as they are dragged together with the datasets they reference.

Hmm, I cannot replicate the bug when dropping narratives with associated datasets.
Based on the example console message, it seems like you had a 2y vs 3y mismatch in the dataset name.

🤦 thanks @joverlee521

#25 should be able to improve this experience