Don't paste if xsel fails

This silences a rather verbose error.

share/functions/fish_clipboard_paste.fish

@@ -2,6 +2,11 @@ function fish_clipboard_paste
     if type -q pbpaste
         commandline -i -- (pbpaste)
     else if type -q xsel
-        commandline -i -- (xsel --clipboard)
+        # Only run `commandline` if `xsel` succeeded.
+        # That way any xsel error is printed (to show e.g. a non-functioning X connection),
+        # but we don't print the redundant (and overly verbose for this) commandline error.
+        if set -l data (xsel --clipboard)
+            commandline -i -- $data
+        end
     end
 end