"watch" command: No longer crash if a build fails.
"watch" command: No longer crash if a build fails.

Now being able to rely on a filter problem raising a FilterError, we can
simply catch the exceptions.

0x1997/webassets,wijerasa/webassets,john2x/webassets,JDeuce/webassets,aconrad/webassets,heynemann/webassets,heynemann/webassets,glorpen/webassets,glorpen/webassets,heynemann/webassets,scorphus/webassets,JDeuce/webassets,john2x/webassets,scorphus/webassets,wijerasa/webassets,aconrad/webassets,florianjacob/webassets,glorpen/webassets,0x1997/webassets,florianjacob/webassets,aconrad/webassets