Meaningless System.Exception thrown if no proxy for unmanaged exception found

[jng]

If an MgException-derived exception is thrown from the C++ code, and the .net wrapper fails to resolve an appropriate .net proxy class equivalent to throw on the managed side. A plain vanilla System.Exception is thrown.

This is not an issue, except that the exception is thrown without a message, making error diagnosis quite unhelpful.

At the very least, this exception should contain the .net class name of the proxy exception class we could not find to throw a new instance of.

[jng]

Fixed r7666