Consolidated Error Handling

[richardpiazza]

Consolidates all of the errors produced by the client into a single enum. Although not entirely needed, the majority of errors that could be potentially thrown were from APIManager.NetworkError. With that being an internal type, that limited the ability of a client consumer from being able to type catch and potentially resolve issues. All errors are now of type FlagsmithError.

Additional notes:

• Moved Router to its own file and added the ability to pass a JSONEncoder to the request method enabling unit tests.
• Refactored the APIManager request method to not need the emptyResponse parameter. The conditional checking for a specific type and flag was a bit cumbersome to follow. Now the completion block dictates if the response is decoded vs just indicating success/failure.
• Replaced the fatalError() in the APIManager with an error completion. Questionable wether the library should cause an application to crash for being misconfigured.
• Added additional documentation comments to fill out automatic DocC documentation that can be generated in Xcode.
• Unit tests for Router and APIManager where possible.

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I know I can go a bit overboard with some changes and opinionated behaviors. Please, feel free to reject or request changes. The company I work for is likely adopting Flagsmith, hence all the PRs!

[dabeeeenster]

This is great - thanks so much