Better missing function handling in loading DSVarArgFunction

This pull request allows us to better handle missing function in ```DSVarArgFunction``` node. The improvements are equally applicable to both ```DYF``` and ```DYN``` files. Prior to this fix, if ```DSVarArgFunction``` points to a missing function, it would result in Dynamo hard crashing with an exception. The fix is of course to turn these unidentified ```DSVarArgFunction``` nodes into ```DummyNode``` while still allowing the rest of the file to be loaded.

This issue was originally caught by our unit test cases and they are now fixed:

```
Dynamo.Tests.CoreTests.CanOpenDSVarArgFunctionFile
Dynamo.Tests.DSEvaluationTest.Regress781
Dynamo.Tests.FileReadingTests.Defect_MAGN_781
```

```MigrationManager.CreateDummyNodeForFunction``` helper method has since been updated to better handle ```DSVarArgFunction``` nodes in addition to ```DSFunction``` nodes. Relevant unit test cases are added to verify this code change.