TypeScript: use unknown to represent error

[OliverJAsh]

WIP.

unknown makes more sense because an error can be any type—we don't know what the type will be.

Currently this would give a type error:

const reportMyError = (error: unknown) => {
  Raven.captureException(error, {})
};

This fixes that.

There are probably other usages of Error that we should replace with unknown. I don't have the time to consider this right now, but I hope to come back to this PR. At the very least, this PR can act as an issue.

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[kamilogorek]

Thanks!

[chuchuva]

This breaks builds if you use TypeScript 2 :(

Cannot find name 'unknown'.