Fix segfault with undefined template declaration errors

As posted on : http://forum.dlang.org/thread/lvpj5j$auh$1@digitalmars.com#post-lvpm16:24dj0:241:40digitalmars.com

This ICE is a segfault when attempting to display what this (fixed dmd) shows as:

`Error: overload alias Array does not match any template declaration`

Which showed up when an invalid type was being used as a template parameter. e.g. `Array!wd` where `wd` is undefined.