Remove conflicting exceptions

[timcassell]

Some exceptions in the Proto.Promises namespace have the same name as exceptions in the System namespace (example ArgumentNullException). Including both namespaces in a file is common, leading to ambiguities when trying to throw one of these exceptions without using the fully qualified name. This was largely done for simple double-click exception messages in Unity's console (the StackTrace property is overridden to ignore internal stack frames), and so that users could still catch the system exception type and see the same type name in its ToString.

We are already using .Net 6's [StackTraceHidden], and Unity is working on adding latest .Net support, so I think the convenience of the console double-click will soon be moot. I also feel it is outweighed by the inconvenience of the ambiguities, and older Unity console can just click the line of the stack frame in the extended message, instead of double-clicking the short message.