Convert reserved character sequence warnings into errors

[bamarsha]

microsoft/qsharp-runtime#349 updated C# generation to avoid name collisions in the generated code, but it relies on new reserved character sequences being added to Q#. #578 added temporary warnings for those identifiers.

• The warnings added by Warning for underscore patterns that will be reserved in the future #578 need to be turned into errors in the September release.
• Data and Item should be removed from the reserved words list.