Allow Exception object in _error struct.

Hi.
I tried to do something like:
```
struct ErrInfo
{
	Exception error;
	string field;
}
void getList(ErrInfo _error) {...}
@errorDisplay!getList
void postNew() {...}
```
ErrorDisplayAttribute.getError allow fill Exception object in _error struct.
But if there is no error information, HandleRequest trying to read _error from  req.params.
And it fails because webConvTo can not handle this struct.
This pull request fixes this. But now _error not be read from req.params at all. And now _error can not have default value.
I think that the _error parameter is enough internally, to behave this way. But I'm not entirely sure.
Thanks!