Added ToString override to ContractError to make it easier to present.
Removed irrelevant code.
Changed the implementation to return the error message if the string could not be parsed as xml. That may not be a good idea in a production scenario, but this is a test environment and we want to be alerted if something is awry.
Added ctor override that takes a simple string as input.
Added tests to check for appropriate creation of Error behavior.
Cleaned up some, and added testst for error handling in PostAsync.