feat(copyfile): add error handling (#13248)

plugins/copyfile/copyfile.plugin.zsh

@@ -1,7 +1,19 @@
 # Copies the contents of a given file to the system or X Windows clipboard
 #
-# copyfile <file>
+# Usage: copyfile <file>
 function copyfile {
   emulate -L zsh
+
+  if [[ -z "$1" ]]; then
+    echo "Usage: copyfile <file>"
+    return 1
+  fi
+
+  if [[ ! -f "$1" ]]; then
+    echo "Error: '$1' is not a valid file."
+    return 1
+  fi
+
   clipcopy $1
+  echo ${(%):-"%B$1%b copied to clipboard."}
 }