API: emit nvim_error_event on failed async request

We already do this for _invalid_ async requests #9300.
Now we also do it for failed invocation of valid requests.

test/helpers.lua

@@ -627,6 +627,19 @@ local function table_flatten(arr)
   return result
 end
 
+-- Checks if a list-like (vector) table contains `value`.
+local function table_contains(t, value)
+  if type(t) ~= 'table' then
+    error('t must be a table')
+  end
+  for _,v in ipairs(t) do
+    if v == value then
+      return true
+    end
+  end
+  return false
+end
+
 local function hexdump(str)
   local len = string.len(str)
   local dump = ""
@@ -771,6 +784,7 @@ local module = {
   repeated_read_cmd = repeated_read_cmd,
   shallowcopy = shallowcopy,
   sleep = sleep,
+  table_contains = table_contains,
   table_flatten = table_flatten,
   tmpname = tmpname,
   uname = uname,