gracefully handle missing model classes in Page.specific()
gracefully handle missing model classes in Page.specific()

inonit/wagtail,mayapurmedia/wagtail,zerolab/wagtail,serzans/wagtail,bjesus/wagtail,nealtodd/wagtail,m-sanders/wagtail,jorge-marques/wagtail,kurtw/wagtail,Toshakins/wagtail,WQuanfeng/wagtail,zerolab/wagtail,mjec/wagtail,WQuanfeng/wagtail,iansprice/wagtail,takeshineshiro/wagtail,Klaudit/wagtail,torchbox/wagtail,nrsimha/wagtail,mixxorz/wagtail,nrsimha/wagtail,jordij/wagtail,kurtw/wagtail,dresiu/wagtail,taedori81/wagtail,chrxr/wagtail,timorieber/wagtail,nimasmi/wagtail,nimasmi/wagtail,mixxorz/wagtail,zerolab/wagtail,WQuanfeng/wagtail,m-sanders/wagtail,zerolab/wagtail,rsalmaso/wagtail,Klaudit/wagtail,FlipperPA/wagtail,Pennebaker/wagtail,mephizzle/wagtail,hanpama/wagtail,nutztherookie/wagtail,thenewguy/wagtail,chrxr/wagtail,wagtail/wagtail,inonit/wagtail,dresiu/wagtail,nrsimha/wagtail,hamsterbacke23/wagtail,rv816/wagtail,dresiu/wagtail,iho/wagtail,mayapurmedia/wagtail,takeflight/wagtail,kurtrwall/wagtail,nimasmi/wagtail,dresiu/wagtail,timorieber/wagtail,serzans/wagtail,Tivix/wagtail,torchbox/wagtail,Toshakins/wagtail,Tivix/wagtail,nilnvoid/wagtail,mikedingjan/wagtail,stevenewey/wagtail,rsalmaso/wagtail,quru/wagtail,tangentlabs/wagtail,takeshineshiro/wagtail,jnns/wagtail,rv816/wagtail,kaedroho/wagtail,FlipperPA/wagtail,mikedingjan/wagtail,gasman/wagtail,taedori81/wagtail,nilnvoid/wagtail,torchbox/wagtail,kurtrwall/wagtail,hamsterbacke23/wagtail,kurtw/wagtail,Toshakins/wagtail,marctc/wagtail,mjec/wagtail,thenewguy/wagtail,hamsterbacke23/wagtail,gasman/wagtail,takeflight/wagtail,gogobook/wagtail,kurtw/wagtail,stevenewey/wagtail,mixxorz/wagtail,tangentlabs/wagtail,timorieber/wagtail,davecranwell/wagtail,rv816/wagtail,nealtodd/wagtail,jnns/wagtail,mikedingjan/wagtail,mayapurmedia/wagtail,hanpama/wagtail,davecranwell/wagtail,hamsterbacke23/wagtail,jorge-marques/wagtail,iansprice/wagtail,taedori81/wagtail,rsalmaso/wagtail,rjsproxy/wagtail,takeshineshiro/wagtail,jordij/wagtail,iansprice/wagtail,KimGlazebrook/wagtail-experiment,rv816/wagtail,wagtail/wagtail,gasman/wagtail,nealtodd/wagtail,rsalmaso/wagtail,dresiu/wagtail,Klaudit/wagtail,mixxorz/wagtail,jordij/wagtail,inonit/wagtail,Toshakins/wagtail,mephizzle/wagtail,JoshBarr/wagtail,nilnvoid/wagtail,Klaudit/wagtail,kaedroho/wagtail,davecranwell/wagtail,JoshBarr/wagtail,quru/wagtail,quru/wagtail,KimGlazebrook/wagtail-experiment,kurtrwall/wagtail,jorge-marques/wagtail,mjec/wagtail,iho/wagtail,taedori81/wagtail,rjsproxy/wagtail,jordij/wagtail,marctc/wagtail,bjesus/wagtail,serzans/wagtail,quru/wagtail,tangentlabs/wagtail,jorge-marques/wagtail,gogobook/wagtail,hanpama/wagtail,chrxr/wagtail,nilnvoid/wagtail,stevenewey/wagtail,thenewguy/wagtail,rjsproxy/wagtail,stevenewey/wagtail,marctc/wagtail,bjesus/wagtail,mjec/wagtail,marctc/wagtail,davecranwell/wagtail,zerolab/wagtail,kurtrwall/wagtail,m-sanders/wagtail,JoshBarr/wagtail,Pennebaker/wagtail,janusnic/wagtail,takeflight/wagtail,gasman/wagtail,darith27/wagtail,janusnic/wagtail,wagtail/wagtail,darith27/wagtail,iho/wagtail,Tivix/wagtail,nealtodd/wagtail,gogobook/wagtail,serzans/wagtail,nutztherookie/wagtail,WQuanfeng/wagtail,rjsproxy/wagtail,rsalmaso/wagtail,kaedroho/wagtail,KimGlazebrook/wagtail-experiment,wagtail/wagtail,nimasmi/wagtail,mixxorz/wagtail,taedori81/wagtail,tangentlabs/wagtail,KimGlazebrook/wagtail-experiment,gogobook/wagtail,nutztherookie/wagtail,bjesus/wagtail,inonit/wagtail,kaedroho/wagtail,thenewguy/wagtail,gasman/wagtail,mephizzle/wagtail,janusnic/wagtail,darith27/wagtail,hanpama/wagtail,FlipperPA/wagtail,janusnic/wagtail,Pennebaker/wagtail,jnns/wagtail,darith27/wagtail,thenewguy/wagtail,nutztherookie/wagtail,iansprice/wagtail,takeflight/wagtail,mephizzle/wagtail,timorieber/wagtail,nrsimha/wagtail,JoshBarr/wagtail,m-sanders/wagtail,iho/wagtail,mayapurmedia/wagtail,torchbox/wagtail,Pennebaker/wagtail,jorge-marques/wagtail,chrxr/wagtail,FlipperPA/wagtail,mikedingjan/wagtail,jnns/wagtail,wagtail/wagtail,takeshineshiro/wagtail,Tivix/wagtail,kaedroho/wagtail