Developers were getting wrapped exceptions which were not very
distinguishable and required a lot of digging to pull out the base
exception which is the most relevant piece.

Fixed #515

Created an ErrorExtensions class to add a "GetError" capability to
Exceptions.  GetError returns an error of type SignalRError which
unwraps and pulls unique attributes out of a thrown exception to expose
essential pieces to errors.

Made the SignalRError object disposable to assure no memory is leaked
via the HttpWebResponse associated with WebExceptions

The prior URL was pointing to localhost and if an IIS server was not
running the machine would actively refuse the connection, resulting in a
failure of the test (not a 404).