wip fix for error handling

[kumavis]

Fixes #22

webextension-polyfill-ts doesn't include the firefox variant with the error appended to it

and other typescript woes

[Gudahtt]

Merging #24 first might help with this. It made my editor more legible at least, because beforehand TypeScript was highlighting the entire function for each error. Now it only highlights the relevant property/line.etc.

[legobeat]

@kumavis @Gudahtt #25 seems related - which of these if any seems preferred today?

[Gudahtt]

I kinda like the idea of injecting the Chrome extension API rather than accessing it as a global.

Not sure why we'd want to use console.warn though. Passing the error up directly lets the caller decide how to handle this. Much more flexible.