Commented Issue: Problem using Evaluate of not defined symbol [34]

Hi,
Thanks first of all for your component. It is really wonderful :-)

i'm using R.Net in both .Net 3.5 (old version) and .Net 4.0 (new version). My code looks like this

```
engine.Evaluate ("xx");
```

If xx is not defined in R, i get an exception in the old version, which is ok. In the new version the whole system is hanging, no reaction, nothing. The last code runs is:

```
Expression::public bool TryEvaluate(REnvironment environment, out SymbolicExpression result)

```
Do you have any idea about this situation.

Thanks in advance,
Mohamed
Comments: Likely solved by work on https://rdotnet.codeplex.com/workitem/77; worth putting in a exception handling unit test.