lsp: Only mute RequestCancelled or ContentModified, but not other errors

handlers passed to `lsp_buf_request` weren't called if the server
responded with an error that looks like this:

     "decoded", {
      error = {
        code = -32601,
        message = "No delegateCommandHandler for foo"
      },
      id = 5,
      jsonrpc = "2.0"
    }

An example where that happens is both eclipse.jdt.ls and the
haskell-language-server when invoking a command that doesn't exist:

    :lua vim.lsp.buf_request(
      0,
      'workspace/executeCommand',
      { command = 'foo' },
      function(err, _, res)
        print(vim.inspect(err), vim.inspect(res))
      end
    )

runtime/lua/vim/lsp/rpc.lua

@@ -523,14 +523,21 @@ local function start(cmd, cmd_args, dispatchers, extra_spawn_params)
       decoded.error = convert_NIL(decoded.error)
       decoded.result = convert_NIL(decoded.result)
 
+      -- We sent a number, so we expect a number.
+      local result_id = tonumber(decoded.id)
+
       -- Do not surface RequestCancelled or ContentModified to users, it is RPC-internal.
       if decoded.error then
+        local mute_error = false
         if decoded.error.code == protocol.ErrorCodes.RequestCancelled then
           local _ = log.debug() and log.debug("Received cancellation ack", decoded)
+          mute_error = true
         elseif decoded.error.code == protocol.ErrorCodes.ContentModified then
           local _ = log.debug() and log.debug("Received content modified ack", decoded)
+          mute_error = true
         end
-        local result_id = tonumber(decoded.id)
+
+        if mute_error then
           -- Clear any callback since this is cancelled now.
           -- This is safe to do assuming that these conditions hold:
           -- - The server will not send a result callback after this cancellation.
@@ -541,9 +548,8 @@ local function start(cmd, cmd_args, dispatchers, extra_spawn_params)
           end
           return
         end
+      end
 
-      -- We sent a number, so we expect a number.
-      local result_id = tonumber(decoded.id)
       local callback = message_callbacks[result_id]
       if callback then
         message_callbacks[result_id] = nil