Descriptive error handling additions

[ShihabMehboob]

Added better error handling (descriptions) to avoid using 'unknown' in errors which could be vague for users, especially those getting the error when the client wasn't connected to the server.

[codecov-io]

Codecov Report

Merging #24 into master will not change coverage. The diff coverage is 0%.

@@           Coverage Diff           @@
##           master      #24   +/-   ##
=======================================
  Coverage   71.96%   71.96%           
=======================================
  Files           2        2           
  Lines         132      132           
=======================================
  Hits           95       95           
  Misses         37       37

Flag	Coverage Δ
#KituraKit	71.96% <0%> (ø)	:arrow_up:

Impacted Files	Coverage Δ
Sources/KituraKit/RequestErrorExtension.swift	27.27% <0%> (ø)	:arrow_up:
Sources/KituraKit/Client.swift	76.03% <0%> (ø)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update 8ce52cd...23da389. Read the comment docs.

[ianpartridge]

These are a definite improvement!

Can we guide the user a bit more about how they might handle the errors if they see them?

[ShihabMehboob]

Sure, added brief guidance on how the user may handle the errors.

[ianpartridge]

Nice.