Better error messages for invalid interfaces.

Now throws InvalidOperationException if the given
service interface:
 * Is not marked public
 * Is not marked with ServiceContract attribute
 * Does not contain methods marked with OperationContract
The error messages are much easier to understand now.
Merge branch 'bettererrors' of https://github.com/varkkjuh/WcfClientProxyGenerator into varkkjuh-bettererrors

Conflicts:
	source/WcfClientProxyGenerator.Tests/ProxyTests.cs