Add support for faking errors for throwing functions

[danielr]

See #2.

• Support in FunctionMock
• Support in ReturningFunctionMock
• Extend example
• Extend documentation

[codecov-io]

Codecov Report

Merging #4 into master will increase coverage by 0.34%.
The diff coverage is 98.82%.

@@            Coverage Diff             @@
##           master       #4      +/-   ##
==========================================
+ Coverage   98.27%   98.62%   +0.34%     
==========================================
  Files           6        6              
  Lines         290      363      +73     
==========================================
+ Hits          285      358      +73     
  Misses          5        5

Impacted Files	Coverage Δ
Mokka/Sources/FunctionMock/FunctionMock.swift	100% <100%> (ø)	⬆️
...ests/FunctionMock/ReturningFunctionMockTests.swift	100% <100%> (ø)	⬆️
Mokka/Tests/FunctionMock/FunctionMockTests.swift	100% <100%> (ø)	⬆️
...a/Sources/FunctionMock/ReturningFunctionMock.swift	97.77% <93.33%> (+0.4%)	⬆️

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