Print unlinked warning to standard error instead of standard out (#459)

Author: cmcgee1024

Sources/Swiftly/Unlink.swift

@@ -71,7 +71,7 @@ extension SwiftlyCommand {
  /// - Parameter ctx: The Swiftly context.
  func validateLinked(_ ctx: SwiftlyCoreContext) async throws {
  if try await !self.isLinked(ctx) {
- await ctx.message(Messages.currentlyUnlinked)
+ await ctx.printError(Messages.currentlyUnlinked)
  }
  }