Add a helpful message if there is a 404 on config.json error

[cmdcolin]

Tries to help with #2171

Based on the idea that a red error box for a 404 not found error might be overly scary, so it specifically detects this case and displays a green message instead of a red one

[codecov]

Codecov Report

Merging #2203 (4e9fced) into main (b5cb5ab) will decrease coverage by 0.01%.
The diff coverage is 86.66%.

@@            Coverage Diff             @@
##             main    #2203      +/-   ##
==========================================
- Coverage   62.56%   62.54%   -0.02%     
==========================================
  Files         484      484              
  Lines       22707    22712       +5     
  Branches     5152     5152              
==========================================
  Hits        14206    14206              
- Misses       8231     8237       +6     
+ Partials      270      269       -1     

Impacted Files	Coverage Δ
products/jbrowse-web/src/Loader.tsx	62.54% <86.66%> (+0.97%)	⬆️
...nts/src/SNPCoverageRenderer/SNPCoverageRenderer.ts	80.24% <0.00%> (-7.41%)	⬇️

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[elliothershberg]

From discussion of this PR in grooming, we opted to link people to our web quickstart guide if no config is found.

[cmdcolin]

looks good

has big "error: operation completed successfully" vibes :)