Fix #2438 - Print a friendly message when the clipboard is not available

autoload/ale/debugging.vim

@@ -238,6 +238,12 @@ function! ale#debugging#Info() abort
 endfunction
 
 function! ale#debugging#InfoToClipboard() abort
+    if !has('clipboard')
+        call s:Echo('clipboard not available. Try :ALEInfoToFile instead.')
+
+        return
+    endif
+
     redir => l:output
         silent call ale#debugging#Info()
     redir END

test/test_ale_info_to_clipboard.vader

@@ -0,0 +1,15 @@
+After:
+  unlet! g:output
+
+Execute(ALEInfoToClipboard should that clipboard support is required):
+  " When run in the Docker image, there's no clipboard support, so this test
+  " will actually run.
+  if !has('clipboard')
+    let g:output = ''
+
+    redir => g:output
+      :ALEInfoToClipboard
+    redir END
+
+    AssertEqual 'clipboard not available. Try :ALEInfoToFile instead.', join(split(g:output))
+  endif