Two minor fixes to error handling

[mindplay-dk]

1. You can't actually test for existence of console in global scope by just doing if (console) - this would error. I've corrected it to test with typeof console !== "undefined" which won't error in the absence of console.

2. Accessing global console and writing to the log is an uncontrollable side-effect that creates problems for me, as I'm actually using the console to output JSON data. To avoid a breaking change, I've kept this behavior, but made it optional: you can now supply an optional onError callback instead, overriding the backwards-compatible default behavior of writing to the console.

[mindplay-dk]

@albell is this project maintained?

Looks like no releases since 2016 - but also no open issues, so that could just mean it's done and the spec hasn't changed.

We can't have side-effects in our project - so if this project is unmaintained (or for some reason you don't want to merge this change) we may need to fork the package.