feat: add typed error hierarchy for fetch failures

Define a wttrin-error condition with children wttrin-invalid-input, wttrin-network-error, wttrin-not-found-error, wttrin-service-error, and wttrin-parse-error, so callers branch on the class of a failure instead of matching message text.

Synchronous paths signal these directly: a nil query and an unknown geolocation provider now raise wttrin-invalid-input. The async fetch path can't signal across its callback, so it tags the error string with the class via a wttrin-error-type text property. The wttrin-error-message-type accessor reads it back, and two-arg callbacks are untouched.

Retyping the classifier also closed two gaps: a missing status and a 2xx with an empty body used to go silent or get mislabeled "Unexpected HTTP status". Both are now parse errors.

wttrin-geolocation.el now requires wttrin for the shared conditions. It's only ever loaded through wttrin, so the require is a no-op in practice and just makes the dependency explicit.

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