job control: reuse common job code for rpc jobs

This makes stderr and exit callbacks work for rpc jobs

runtime/autoload/provider/ruby.vim

@@ -4,6 +4,17 @@ if exists('g:loaded_ruby_provider')
 endif
 let g:loaded_ruby_provider = 1
 
+let s:stderr = {}
+let s:job_opts = {'rpc': v:true}
+
+function! s:job_opts.on_stderr(chan_id, data, event)
+  let stderr = get(s:stderr, a:chan_id, [''])
+  let last = remove(stderr, -1)
+  let a:data[0] = last.a:data[0]
+  call extend(stderr, a:data)
+  let s:stderr[a:chan_id] = stderr
+endfunction
+
 function! provider#ruby#Detect() abort
   return exepath('neovim-ruby-host')
 endfunction
@@ -13,7 +24,7 @@ function! provider#ruby#Prog()
 endfunction
 
 function! provider#ruby#Require(host) abort
-  let args = []
+  let args = [provider#ruby#Prog()]
   let ruby_plugins = remote#host#PluginsForHost(a:host.name)
 
   for plugin in ruby_plugins
@@ -21,13 +32,16 @@ function! provider#ruby#Require(host) abort
   endfor
 
   try
-    let channel_id = rpcstart(provider#ruby#Prog(), args)
+    let channel_id = jobstart(args, s:job_opts)
     if rpcrequest(channel_id, 'poll') ==# 'ok'
       return channel_id
     endif
   catch
     echomsg v:throwpoint
     echomsg v:exception
+    for row in get(s:stderr, channel_id, [])
+      echomsg row
+    endfor
   endtry
   throw remote#host#LoadErrorForHost(a:host.orig_name, '$NVIM_RUBY_LOG_FILE')
 endfunction