Remove `eprintln!` usage from this library? about adblock-rust

[antonok-edm]

@ArniDagur I agree, better methods of error logging sound like a great idea and I'd be happy to improve adblock-rust in this regard.

As it stands, many of the Engine APIs either return nothing, or are builder-style methods that return another mutable reference to self. I think the best way to go about this would be:

• For methods that return nothing, return a Result<(), E> instead, where E is a custom error type denoting the problems that can occur for that particular method.
• For builder-style methods, it'd be impractical to return a Result. I'm not sure how commonly these are actually chained together, but I'm not opposed to doing away with the builder-style altogether. Then these can fit in with the first case as well. The builder-style methods are gone as of v0.3.

Does that sound reasonable?

[ArniDagur]

@AndriusA Hey! I have time over the weekend to work on this. Just tell me how you want it to be done, if at all.

[antonok-edm]

Just pulled out the last few eprintln cases with 88c1a6b, so I'll consider this fixed!