fix(vim.system): unclear non-executable message #33455

Problem:
When a command is not found or not executable, the error message gives
no indication about what command was actually tried.

Solution:
Always append the command name to the error message.

BEFORE:

    E5108: Error executing lua …/_system.lua:248: ENOENT: no such file or directory

AFTER:

    E5108: Error executing lua …/_system.lua:249: ENOENT: no such file or directory: "foo"

fix #33445

runtime/lua/vim/_system.lua

@@ -245,7 +245,7 @@ local function spawn(cmd, opts, on_exit, on_error)
   local handle, pid_or_err = uv.spawn(cmd, opts, on_exit)
   if not handle then
     on_error()
-    error(pid_or_err)
+    error(('%s: "%s"'):format(pid_or_err, cmd))
   end
   return handle, pid_or_err --[[@as integer]]
 end

test/functional/lua/system_spec.lua

@@ -53,6 +53,13 @@ describe('vim.system', function()
 
   for name, system in pairs { sync = system_sync, async = system_async } do
     describe('(' .. name .. ')', function()
+      it('failure modes', function()
+        t.matches(
+          'ENOENT%: no such file .*: "non%-existent%-cmd"',
+          t.pcall_err(system, { 'non-existent-cmd', 'arg1', 'arg2' }, { text = true })
+        )
+      end)
+
       it('can run simple commands', function()
         eq('hello\n', system({ 'echo', 'hello' }, { text = true }).stdout)
       end)