Module errors

Typed error hierarchy for B2 API failures.

Every B2 error response is mapped to a specific B2Error subclass (e.g. ExpiredAuthTokenError, CapExceededError) with pre-computed retryable flags. Use classifyError to convert a raw error response into the appropriate subclass.

Convention: B2Error and its subclasses represent failures returned by the B2 API. Programming errors and SDK preconditions (e.g. "not yet authorized", "stream consumed twice", "called before init") use the native Error constructor instead. Two outliers extend Error directly rather than B2Error because they originate inside the SDK before any B2 request is made: B2InsufficientCapabilityError (raised by B2Client.hasCapabilities) and B2SsrfError (raised by the default UrlGuard before fetch).