fix(routing): return 404 when no record is found

[hacdias]

Addresses ipfs/someguy#58.

Sets 404 if the router returns routing.ErrNotFound or if the list of providers is empty.

[codecov]

Codecov Report

Attention: Patch coverage is 57.14286% with 12 lines in your changes are missing coverage. Please review.

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@@           Coverage Diff           @@
##             main     #601   +/-   ##
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  Coverage        ?   59.47%           
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  Files           ?      233           
  Lines           ?    28294           
  Branches        ?        0           
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  Hits            ?    16829           
  Misses          ?     9992           
  Partials        ?     1473           

Files	Coverage Δ
routing/http/server/server.go	71.85% <57.14%> (ø)

[lidel]

Lgtm, thanks for fixing this, 404s are cachable, 500 are not, so this will save resources across all deployments.

Fine to ship, once small nit below is addressed.