Improve error message when parsing of optional enum option fails

[dlemstra]

When the parsing of an optional parameter that is an enum fails the following error message is printed:

Error: Option 'foo' requires Nullable`1 value. 'bar' cannot be converted to Nullable`1 value.

It would be nice if the Nullable would be removed and the type of the enumeration would be shown instead. The same could be said for int?. Can this error message be improved?