[bugfix] Improve error messages if a document could not be read via fn:doc().
[bugfix] Improve error messages if a document could not be read via fn:doc().

svn path=/trunk/eXist/; revision=11312

ljo/exist,eXist-db/exist,shabanovd/exist,eXist-db/exist,adamretter/exist,shabanovd/exist,ambs/exist,opax/exist,kohsah/exist,ambs/exist,lcahlander/exist,joewiz/exist,patczar/exist,wolfgangmm/exist,jensopetersen/exist,jensopetersen/exist,opax/exist,lcahlander/exist,ambs/exist,kohsah/exist,ambs/exist,windauer/exist,ljo/exist,RemiKoutcherawy/exist,windauer/exist,dizzzz/exist,joewiz/exist,jessealama/exist,ljo/exist,zwobit/exist,adamretter/exist,wshager/exist,jessealama/exist,hungerburg/exist,olvidalo/exist,windauer/exist,MjAbuz/exist,jessealama/exist,shabanovd/exist,eXist-db/exist,wolfgangmm/exist,dizzzz/exist,lcahlander/exist,kohsah/exist,windauer/exist,MjAbuz/exist,olvidalo/exist,joewiz/exist,eXist-db/exist,patczar/exist,adamretter/exist,opax/exist,joewiz/exist,dizzzz/exist,shabanovd/exist,wshager/exist,eXist-db/exist,MjAbuz/exist,dizzzz/exist,lcahlander/exist,patczar/exist,wshager/exist,jensopetersen/exist,eXist-db/exist,wshager/exist,shabanovd/exist,patczar/exist,lcahlander/exist,patczar/exist,wolfgangmm/exist,joewiz/exist,RemiKoutcherawy/exist,zwobit/exist,olvidalo/exist,jessealama/exist,ljo/exist,olvidalo/exist,hungerburg/exist,shabanovd/exist,lcahlander/exist,wshager/exist,MjAbuz/exist,zwobit/exist,windauer/exist,dizzzz/exist,jensopetersen/exist,ambs/exist,opax/exist,windauer/exist,adamretter/exist,zwobit/exist,patczar/exist,wshager/exist,olvidalo/exist,wolfgangmm/exist,jessealama/exist,dizzzz/exist,RemiKoutcherawy/exist,RemiKoutcherawy/exist,hungerburg/exist,adamretter/exist,opax/exist,ljo/exist,kohsah/exist,ambs/exist,kohsah/exist,joewiz/exist,jensopetersen/exist,wolfgangmm/exist,zwobit/exist,RemiKoutcherawy/exist,ljo/exist,hungerburg/exist,MjAbuz/exist,hungerburg/exist,RemiKoutcherawy/exist,zwobit/exist,adamretter/exist,MjAbuz/exist,kohsah/exist,wolfgangmm/exist,jensopetersen/exist,jessealama/exist