Create init for error receiving an Error + Remove unknown error

[ronanrodrigo]

Created an init for FrisbeError and remove some error handling in NetworkGet. With that is more easy to test error handling and .unknown error is unnecessary.

[codecov-io]

Codecov Report

Merging #15 into master will increase coverage by 2.49%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #15      +/-   ##
==========================================
+ Coverage   95.03%   97.53%   +2.49%     
==========================================
  Files           8       12       +4     
  Lines         141      162      +21     
  Branches       11       11              
==========================================
+ Hits          134      158      +24     
+ Misses          7        4       -3

Impacted Files	Coverage Δ
...ces/Frisbee/Factories/ResultGeneratorFactory.swift	100% <100%> (ø)
Sources/Frisbee/Requestables/NetworkGet.swift	100% <100%> (+7.31%)	⬆️
...rces/Frisbee/Interactors/URLWithQueryBuilder.swift	86.66% <100%> (ø)	⬆️
Sources/Frisbee/Entities/FrisbeeError.swift	100% <100%> (ø)	⬆️
Sources/Frisbee/Entities/FrisbeeJSONDecoder.swift	100% <100%> (ø)
...es/Frisbee/Factories/FrisbeeDecodableFacotry.swift	100% <100%> (ø)
...isbee/Factories/URLWithQueryBuildableFactory.swift	100% <100%> (ø)
Sources/Frisbee/Interactors/ResultGenerator.swift	100% <100%> (ø)	⬆️
... and 2 more

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