Fix crash on launching a deleted quickstart

[cmdcolin]

Fixes #2393

Catches errors in a nicer way, and filters out deleted quickstarts before trying to access them

There is a tension here between the observability, and fetching from the filesystem. We are not using mobx here, and since it is corresponding with the filesystem, and maybe even multiple instances of jbrowse, it wouldn't really be automatically solved by mobx, so this is just a best effort here to remain alive and consistent with the filesystem (again, it uses polling)

[codecov]

Codecov Report

Merging #2395 (8f7be08) into main (1870684) will increase coverage by 0.01%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##             main    #2395      +/-   ##
==========================================
+ Coverage   60.86%   60.88%   +0.01%     
==========================================
  Files         534      534              
  Lines       24634    24638       +4     
  Branches     5793     5794       +1     
==========================================
+ Hits        14994    15001       +7     
+ Misses       9324     9321       -3     
  Partials      316      316              

Impacted Files	Coverage Δ
...browse-desktop/src/StartScreen/QuickstartPanel.tsx	0.00% <0.00%> (ø)
...nts/src/SNPCoverageRenderer/SNPCoverageRenderer.ts	80.24% <0.00%> (-7.41%)	⬇️
packages/core/util/index.ts	79.84% <0.00%> (ø)
packages/core/assemblyManager/assemblyManager.ts	68.42% <0.00%> (+1.31%)	⬆️
packages/core/TextSearch/TextSearchManager.ts	100.00% <0.00%> (+3.44%)	⬆️
...inearGenomeView/components/RefNameAutocomplete.tsx	88.04% <0.00%> (+11.95%)	⬆️

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