Improved error handling

[OscarVanL]

Fixes my issue: #13

• Embeds an ErrStatus struct, which contains all of the Rocket.Chat error response fields that can be populated under different scenarios.

• The error handling approach is inspired by how Rocket.Chat's Go SDK handles REST errors, but extended to handle a couple of extra error response fields I have come across and add informative error messages.

• Adds an option WithDebug, which will also include the details field which is sometimes present in error responses. I didn't include this by default because you can get errors that could expose data in logs you didn't want to, like this for validation error's inputs:

{
        "errorType": "error-input-is-not-a-valid-field",
	"details": {
		"method": "insertOrUpdateUser",
		"input": "sdfsdf ;;; ; ; -- ; ; fsdf",
		"field": "Username"
	}
}

• I also fixed up some function comments to be properly formatted Godoc docstrings.

• This could be considered a breaking change if some consumers of this package want request errors to fail without returning an err, but as this is a v0 package we're fine to make breaking changes.

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