v2/handler: add explicit error for uhandled frame type
v2/handler: add explicit error for uhandled frame type

sasa233/tchannel,benfleis/tchannel,bunnyblue/tchannel,davewhat/tchannel,bunnyblue/tchannel,bunnyblue/tchannel,vanloswang/tchannel,Zirpon/tchannel,sasa233/tchannel,chenwenbin928/tchannel,RyanTech/tchannel,benfleis/tchannel,savaki/tchannel,i/tchannel,hustxiaoc/tchannel,benfleis/tchannel,vanloswang/tchannel,RyanTech/tchannel,i/tchannel,chenwenbin928/tchannel,i/tchannel,vanloswang/tchannel,hustxiaoc/tchannel,davewhat/tchannel,sasa233/tchannel,savaki/tchannel,RyanTech/tchannel,chenwenbin928/tchannel,savaki/tchannel,Zirpon/tchannel,hustxiaoc/tchannel,Zirpon/tchannel