Add typespec for Phoenix.Template.template_not_found/2
Add typespec for Phoenix.Template.template_not_found/2

Without this typespec, running Dialyzer on fresh Phoenix project gives
following warnings:
`layout_view.ex:2: Function template_not_found/2 only terminates with explicit exception`
`page_view.ex:2: Function template_not_found/2 only terminates with explicit exception`

evax/phoenix,nshafer/phoenix,stevedomin/phoenix,ericmj/phoenix,Joe-noh/phoenix,Gazler/phoenix,cas27/phoenix,gjaldon/phoenix,rafbgarcia/phoenix,jwarwick/phoenix,phoenixframework/phoenix,iStefo/phoenix,mschae/phoenix,michalmuskala/phoenix,0x00evil/phoenix,phoenixframework/phoenix,keathley/phoenix,eksperimental/phoenix,wsmoak/phoenix,optikfluffel/phoenix,eksperimental/phoenix,Kosmas/phoenix,gjaldon/phoenix,nshafer/phoenix,optikfluffel/phoenix,korobkov/phoenix,rafbgarcia/phoenix,Gazler/phoenix,Ben-Evolently/phoenix,cas27/phoenix,michalmuskala/phoenix,wsmoak/phoenix,mobileoverlord/phoenix,Kosmas/phoenix,trarbr/phoenix,ephe-meral/phoenix,pap/phoenix,ybur-yug/phoenix,dmarkow/phoenix,0x00evil/phoenix,bsmr-erlang/phoenix,iStefo/phoenix,mitchellhenke/phoenix,vic/phoenix,jamilabreu/phoenix,bsmr-erlang/phoenix,stevedomin/phoenix,jwarwick/phoenix,korobkov/phoenix,vic/phoenix,dmarkow/phoenix,evax/phoenix,ephe-meral/phoenix,Joe-noh/phoenix,nshafer/phoenix,mobileoverlord/phoenix,mschae/phoenix,CultivateHQ/phoenix,phoenixframework/phoenix,trarbr/phoenix,CultivateHQ/phoenix,jamilabreu/phoenix,0x00evil/phoenix,optikfluffel/phoenix,mitchellhenke/phoenix,keathley/phoenix,rafbgarcia/phoenix,ybur-yug/phoenix,pap/phoenix,ericmj/phoenix