Cask: refuse to trash root-owned files

These files cannot be trashed from CLI without sudo, which trashes them
instead to root's Trash. AppleScript isn't of use here because it will
break Travis (as it will fail when asking for admin permissions).

Fixes homebrew/homebrew-cask#69897

Library/Homebrew/cask/artifact/abstract_uninstall.rb

@@ -362,6 +362,8 @@ module Cask
       def trash_paths(*paths, command: nil, **_)
         return if paths.empty?
 
+        raise CaskError, "Some files are owned by `root` and cannot be moved to the user's Trash." unless paths.all?(&:writable?)
+
         result = command.run!("/usr/bin/swift", args: [TRASH_SCRIPT, *paths])
 
         # Remove AppleScript's automatic newline.

Library/Homebrew/cask/utils/trash.swift

@@ -2,6 +2,11 @@
 
 import Foundation
 
+struct swifterr: TextOutputStream {
+  public static var stream = swifterr()
+  mutating func write(_ string: String) { fputs(string, stderr) }
+}
+
 if (CommandLine.arguments.count < 2) {
   exit(2)
 }
@@ -12,10 +17,11 @@ for item in CommandLine.arguments[1...] {
   do {
     let path: URL = URL(fileURLWithPath: item)
     try manager.trashItem(at: path, resultingItemURL: nil)
-    print(path)
+    print(path, terminator: "\0")
   }
   catch {
-    print("\0")
+    print(error.localizedDescription, to: &swifterr.stream)
+    exit(1)
   }
 }