IE8 error 'Access Denied'

I changed getDoc to be less error prone while drilling down to the lowest object usable as the "response document". The existing logic of this method is unchanged except it returns a null object and not an exception. I changed the calling code to account for this.

Currently if, an 'Access Denied' exception is thrown from this method, the process will abort. This change allows error independent checking from contentWindow.document -> contentDocument -> document; if all fail a null is returned.

I was able to reproduce this in our company test environment running IE8 over SSL using our corporate enforcement policy's applied to the browser, i.e., ProtectedMode, some various IE8 security settings, not sure what else.