Provide a more sensible error in devserver when a watcher isn't found
Provide a more sensible error in devserver when a watcher isn't found

Popen raises an ugly OSError that just says "[Errno 2] No such file or
directory" with no indication of what or where this came from.

mvaled/sentry,looker/sentry,beeftornado/sentry,gencer/sentry,BuildingLink/sentry,mvaled/sentry,jean/sentry,BuildingLink/sentry,mvaled/sentry,looker/sentry,JamesMura/sentry,mitsuhiko/sentry,mvaled/sentry,JamesMura/sentry,daevaorn/sentry,BuildingLink/sentry,JackDanger/sentry,JamesMura/sentry,zenefits/sentry,gencer/sentry,ifduyue/sentry,looker/sentry,looker/sentry,ifduyue/sentry,jean/sentry,JackDanger/sentry,mvaled/sentry,gencer/sentry,fotinakis/sentry,zenefits/sentry,JackDanger/sentry,ifduyue/sentry,JamesMura/sentry,JamesMura/sentry,mvaled/sentry,mitsuhiko/sentry,ifduyue/sentry,fotinakis/sentry,beeftornado/sentry,zenefits/sentry,zenefits/sentry,fotinakis/sentry,zenefits/sentry,looker/sentry,alexm92/sentry,ifduyue/sentry,fotinakis/sentry,alexm92/sentry,BuildingLink/sentry,jean/sentry,jean/sentry,daevaorn/sentry,daevaorn/sentry,beeftornado/sentry,BuildingLink/sentry,daevaorn/sentry,alexm92/sentry,gencer/sentry,jean/sentry,gencer/sentry