Make loading DSFunction more fault tolerant for unresolved function

##### Make loading DSFunction more fault tolerant for unresolved function

- Introduced a new ```UnresolvedFunctionException``` exception type
- Throws ```UnresolvedFunctionException``` exception if ```DSFunction.LoadNode``` is not able to get the ```FunctionDescriptor``` for a given function signature (e.g. name changes)
- Implemented ```MigrationManager.CreateDummyNodeForFunction``` method
- Added 13 test cases to verify ```MigrationManager.CreateDummyNodeForFunction``` method
- Updated deprecated icon for use in dummy node