fix error-handling in worker

[bryangingechen]

This is a first attempt at addressing part 2 of #279, which points out that errors in the worker builds are not caught.

[ShashkovS]

This also fix error handling in case of syntax error.
Just run request seERRORlect * from table and exception will be lost.
It is because throw in promises works a little bit strange if it is not catched.
https://stackoverflow.com/questions/30715367/why-can-i-not-throw-inside-a-promise-catch-handler
https://stackoverflow.com/questions/39992417/how-to-bubble-a-web-worker-error-in-a-promise-via-worker-onerror

[bryangingechen]

@lovasoa Any comments on this?

[lovasoa]

Sorry for not taking care of this earlier. This looks good to me.