Improved error message for thread-local class instance wrongly initialized

The existing error message can give the impression that the thread-local class instance itself must be const or immutable, not its initial value.  This revised version aims to make clearer what is wrong and how to solve it.

Background to this patch: http://forum.dlang.org/post/mailman.408.1386663198.3242.digitalmars-d-learn@puremagic.com