Add basic reporting of handler errors
Add basic reporting of handler errors

This isn't very nice but it's very helpful for debugging :-)
Improve it later.

ptitjano/bokeh,dennisobrien/bokeh,timsnyder/bokeh,aavanian/bokeh,rs2/bokeh,clairetang6/bokeh,timsnyder/bokeh,Karel-van-de-Plassche/bokeh,DuCorey/bokeh,maxalbert/bokeh,justacec/bokeh,percyfal/bokeh,mindriot101/bokeh,KasperPRasmussen/bokeh,bokeh/bokeh,Karel-van-de-Plassche/bokeh,msarahan/bokeh,jakirkham/bokeh,ptitjano/bokeh,azjps/bokeh,ericmjl/bokeh,KasperPRasmussen/bokeh,clairetang6/bokeh,bokeh/bokeh,schoolie/bokeh,timsnyder/bokeh,mindriot101/bokeh,philippjfr/bokeh,draperjames/bokeh,draperjames/bokeh,phobson/bokeh,dennisobrien/bokeh,percyfal/bokeh,aiguofer/bokeh,jakirkham/bokeh,ericmjl/bokeh,quasiben/bokeh,Karel-van-de-Plassche/bokeh,ptitjano/bokeh,draperjames/bokeh,htygithub/bokeh,mindriot101/bokeh,maxalbert/bokeh,stonebig/bokeh,bokeh/bokeh,Karel-van-de-Plassche/bokeh,rs2/bokeh,ericmjl/bokeh,msarahan/bokeh,DuCorey/bokeh,justacec/bokeh,percyfal/bokeh,aavanian/bokeh,azjps/bokeh,aavanian/bokeh,schoolie/bokeh,percyfal/bokeh,schoolie/bokeh,dennisobrien/bokeh,rs2/bokeh,ericmjl/bokeh,ptitjano/bokeh,schoolie/bokeh,percyfal/bokeh,ericmjl/bokeh,mindriot101/bokeh,rs2/bokeh,philippjfr/bokeh,aavanian/bokeh,gpfreitas/bokeh,phobson/bokeh,jakirkham/bokeh,gpfreitas/bokeh,azjps/bokeh,phobson/bokeh,phobson/bokeh,DuCorey/bokeh,msarahan/bokeh,KasperPRasmussen/bokeh,philippjfr/bokeh,stonebig/bokeh,aavanian/bokeh,maxalbert/bokeh,DuCorey/bokeh,timsnyder/bokeh,stonebig/bokeh,aiguofer/bokeh,philippjfr/bokeh,maxalbert/bokeh,bokeh/bokeh,jakirkham/bokeh,bokeh/bokeh,draperjames/bokeh,ptitjano/bokeh,aiguofer/bokeh,azjps/bokeh,DuCorey/bokeh,gpfreitas/bokeh,philippjfr/bokeh,quasiben/bokeh,phobson/bokeh,gpfreitas/bokeh,KasperPRasmussen/bokeh,dennisobrien/bokeh,dennisobrien/bokeh,htygithub/bokeh,htygithub/bokeh,aiguofer/bokeh,clairetang6/bokeh,KasperPRasmussen/bokeh,rs2/bokeh,azjps/bokeh,justacec/bokeh,draperjames/bokeh,clairetang6/bokeh,stonebig/bokeh,quasiben/bokeh,aiguofer/bokeh,msarahan/bokeh,timsnyder/bokeh,schoolie/bokeh,htygithub/bokeh,jakirkham/bokeh,justacec/bokeh,Karel-van-de-Plassche/bokeh