[ios, macos] Refined source removal errors

Clarified and copyedited the descriptions of errors that are raised when attempting to remove a source from a style. Worded the changelog entry more generically because the same method in Swift throws an error instead of returning.

A mismatch would occur if the identifier is used by a different source than the one being removed. An application could cause this error to occur by keeping a reference to an MGLSource after changing the style URL of an MGLMapView. Another way would be to remove a source from the style, add another source to the style with the same identifier, then attempt to re-remove the original source from the style.

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