provider: Extra pythonx's stderr handling to common functions

runtime/autoload/provider.vim

@@ -0,0 +1,18 @@
+" Common functionality for providers
+
+let s:stderr = {}
+
+function! provider#stderr_collector(chan_id, data, event) dict
+   let stderr = get(s:stderr, a:chan_id, [''])
+   let stderr[-1] .= a:data[0]
+   call extend(stderr, a:data[1:])
+   let s:stderr[a:chan_id] = stderr
+endfunction
+
+function! provider#clear_stderr(chan_id)
+   silent! call delete(s:stderr, a:chan_id)
+endfunction
+
+function! provider#get_stderr(chan_id)
+   return get(s:stderr, a:chan_id, [])
+endfunction