Promise code lighter; errors thrown are shown.

The error not shown root cause was bare `return` in last promise,
now there is `resolve(true); return`
'Compile Time' only in verbose; finished_callback checked for null
Fixed empty program & resolve in com_js_fil (by mkroehnert)
Merge pull request #816 from herby/promises-simpler-error-shown

Promise code lighter; errors thrown are shown.