clipboard: check for X on every invocation

xsel and xcopy may be available even if a valid X display is not. Also,
the availability of X may change at any time, so check on each
invocation.

Closes #1509.

runtime/autoload/provider/clipboard.vim

@@ -4,6 +4,15 @@
 let s:copy = ''
 let s:paste = ''
 
+function! s:try_cmd(cmd, ...)
+  let out = a:0 ? systemlist(a:cmd, a:1) : systemlist(a:cmd)
+  if v:shell_error
+    echo "clipboard: error: ".(len(out) ? out[0] : '')
+    return ''
+  endif
+  return out
+endfunction
+
 if executable('pbcopy')
   let s:copy = 'pbcopy'
   let s:paste = 'pbpaste'
@@ -13,28 +22,21 @@ elseif executable('xsel')
 elseif executable('xclip')
   let s:copy = 'xclip -i -selection clipboard'
   let s:paste = 'xclip -o -selection clipboard'
-endif
-
-if s:copy == ''
-  echom 'No shell command for communicating with the clipboard found.'
+else
+  echom 'clipboard: No shell command for communicating with the clipboard found.'
   finish
 endif
 
-let s:methods = {}
+let s:clipboard = {}
 
-function! s:ClipboardGet(...)
-  return systemlist(s:paste)
+function! s:clipboard.get(...)
+  return s:try_cmd(s:paste)
 endfunction
 
-function! s:ClipboardSet(...)
-  call systemlist(s:copy, a:1)
+function! s:clipboard.set(...)
+  call s:try_cmd(s:copy, a:1)
 endfunction
 
-let s:methods = {
-      \ 'get': function('s:ClipboardGet'),
-      \ 'set': function('s:ClipboardSet')
-      \ }
-
 function! provider#clipboard#Call(method, args)
-  return s:methods[a:method](a:args)
+  return s:clipboard[a:method](a:args)
 endfunction