fix: throw ResponseError instead of NSError

Sources/SourceKitLSP/Swift/SwiftLanguageService.swift

@@ -149,7 +149,7 @@ public actor SwiftLanguageService: LanguageService, Sendable {
   var documentManager: DocumentManager {
     get throws {
       guard let sourceKitLSPServer = self.sourceKitLSPServer else {
-        throw NSError(domain: "SourceKitLSPErrorDomain", code: 1001, userInfo: [NSLocalizedDescriptionKey: "Cannot get document manager because sourceKitLSPServer was deallocated"])
+        throw ResponseError.unknown("Connection to the editor closed")
       }
       return sourceKitLSPServer.documentManager
     }