Allow HBApplication.start() to throw errors

[adam-fowler]

For some reason I had it ignoring errors. This has been rectified

[codecov]

Codecov Report

Merging #73 (5583f0c) into main (26fc603) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main      #73   +/-   ##
=======================================
  Coverage   82.84%   82.84%           
=======================================
  Files          52       52           
  Lines        2500     2501    +1     
=======================================
+ Hits         2071     2072    +1     
  Misses        429      429           

Impacted Files	Coverage Δ
Sources/Hummingbird/Application.swift	92.72% <100.00%> (ø)
Sources/HummingbirdXCT/Application+XCT.swift	100.00% <100.00%> (ø)
Sources/HummingbirdXCT/HBXCTLive.swift	90.00% <100.00%> (ø)
...Hummingbird/Server/Application+HTTPResponder.swift	100.00% <0.00%> (ø)
Sources/Hummingbird/Server/Request.swift	94.28% <0.00%> (+0.16%)	⬆️

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