refactor: flatten nested if-else blocks

[cades]

apply "return early, avoid else" practice to make code simple and easy to read

[developit]

Need to check the effect on filesize, but I'd imagine this is a savings! (due to uglify better optimizing early returns than nested conditionals)

[developit]

hey @cades - that nested .then() is important - without it, an exception thrown by the method being called won't be passed back via RPC, it'll just be emitted as an error in the worker.

[cades]

@developit thanks for point that out! Seems my knowledge to Promise is not promising 😂 BTW, if you think it is OK to accept this PR, I'm willing to squash these commits into a single commit before merge.