Use LocalizedError and not Error in swift, and override errorDescription

[thomcc]

This make the firefox-ios sentry errors a little easier to deal with.

Fixes #853.

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[garvankeeley]

A thousand thank yous for putting up with the verbosity of Swift custom error type descriptions.

[nolanamy]

All my Sentry errors looked like "The operation couldn’t be completed..."

A thousand thank yous for the fix.