Added custom SwiftSyft errors

[mjjimenez]

Description

Added custom swift syft errors as enums for easier handling. Closes #168 .

Checklist

• [x] I have followed the Contribution Guidelines and Code of Conduct
• [x] I have commented my code following the OpenMined Styleguide
• [x] I have labeled this PR with the relevant Type labels
• [x] My changes are covered by tests

[codecov[bot]]

Codecov Report

Merging #178 into dev will increase coverage by 1.57%. The diff coverage is 63.09%.

@@            Coverage Diff             @@
##              dev     #178      +/-   ##
==========================================
+ Coverage   52.97%   54.55%   +1.57%     
==========================================
  Files          16       17       +1     
  Lines        1882     1921      +39     
==========================================
+ Hits          997     1048      +51     
+ Misses        885      873      -12     

Impacted Files	Coverage Δ
SwiftSyft/Classes/SyftPlan.swift	7.54% <0.00%> (ø)
SwiftSyft/Classes/SwiftSyftError.swift	23.80% <23.80%> (ø)
SwiftSyft/Classes/SignallingMessages.swift	35.03% <50.00%> (ø)
SwiftSyft/Classes/SyftClient.swift	81.44% <80.00%> (+2.72%)	:arrow_up:
SwiftSyft/Classes/NetworkManager.swift	73.07% <100.00%> (ø)
SwiftSyft/Classes/SignallingClient.swift	87.27% <0.00%> (+16.36%)	:arrow_up:
SwiftSyft/Classes/SyftWebSocket.swift	22.03% <0.00%> (+22.03%)	:arrow_up:

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