Error banner

[frasercl]

Estimated review time: moderate (~20-30min)

Resolves #238: adds a banner to display errors encountered during data loading.

The banner includes:

• A collapsible description, which may be one of a few instructive messages based on the type field of VolumeLoadError (introduced in allen-cell-animated/volume-viewer#210)
• The ability to display multiple errors one at a time
• Tracking for which error messages you've seen before - i.e. if you've already gotten the error message "foobar" once before, the next time it shows up it will look like "foobar (2)"

To test

Getting multiple errors as I did in some of the screenshots below is hard without deliberately injecting your own errors, but these links will produce various (single) errors:

• Volume too big: http://localhost:9020/viewer?url=https://janelia-flylight-imagery-dev.s3.amazonaws.com/Fly-eFISH/NP01_1_1_SS00790_AstA546_CCHa1_647_1x_LOL.chunked.zarr/0
• Volume metadata bad: http://localhost:9020/viewer?url=https://uk1s3.embassy.ebi.ac.uk/idr/zarr/v0.2/idr0070A/9838562.zarr
• Incompatible multi-source Zarr: http://localhost:9020/viewer?url=https://animatedcell-test-data.s3.us-west-2.amazonaws.com/variance/1.zarr,https://animatedcell-test-data.s3.us-west-2.amazonaws.com/20200323_F01_001/P13-C4.zarr/

Screenshots!

[frasercl]

Per @lynwilhelm's feedback, the banner is now more notification-like, with all the same behavior in a thinner, floatier package: