Replace the vague `playServicesError` catch-all with seven dedicated
error codes (`playStoreNotFound`, `playServicesNotFound`,
`cannotBindToService`, `playStoreVersionOutdated`,
`playServicesVersionOutdated`, `clientTransientError`, `appNotOwned`)
and add `internalError`, giving every `AgeSignalsErrorCode` value its
own corresponding `AgeVerificationErrorCode`. Fix a test typo
(`NOT_INITIALISED` → `NOT_INITIALIZED`). Bump version to 0.2.6.

BREAKING CHANGE: `AgeVerificationErrorCode.playServicesError` removed.