fix: Add description for ClientError (#2)

Author: RemiBardon

Sources/ProseCoreFFI/Errors+Description.swift

@@ -0,0 +1,16 @@
+//
+// This file is part of prose-app-macos.
+// Copyright (c) 2023 Prose Foundation
+//
+
+import Foundation
+
+extension ClientError: LocalizedError {
+  /// - Important: ``ClientError`` is not localized.
+  public var errorDescription: String? {
+    switch self {
+    case .Generic(let msg):
+      return msg
+    }
+  }
+}