AssertionError after parseHay encounters syntax errors

[PossiblyAShrub]

From Alexey Shamrin on Zulip (#oils-discuss > Segmentation fault in evalHay after some syntax errors),

Seems like a bug:

$ cat crash.hay 
echo =
$ cat crash.ysh 
const h = parseHay('crash.hay')
const result = evalHay(h)
$ ysh --version
Oils 0.37.0             https://oils.pub/

git commit = a9e1764bc62097e63e385a631a476c04bce534e8
$ ysh crash.ysh 
  echo =
       ^
crash.hay:1: Unexpected = (Hint: use var/setvar, or quote it)
Segmentation fault

When run in a python build we see the AssertionError:

ysh-0.37$ = parseHay('crash.hay')
  echo =
       ^
crash.hay:1: Unexpected = (Hint: use var/setvar, or quote it)
Traceback (most recent call last):
  File "/home/aolse/oils/oil/bin/oils_for_unix.py", line 263, in <module>
    sys.exit(main(sys.argv))
  File "/home/aolse/oils/oil/bin/oils_for_unix.py", line 232, in main
    return AppBundleMain(argv)
  File "/home/aolse/oils/oil/bin/oils_for_unix.py", line 186, in AppBundleMain
    return shell.Main(lang, arg_r, environ, login_shell, loader, readline)
  File "/home/aolse/oils/oil/core/shell.py", line 1214, in Main
    prompt_plugin, waiter, errfmt)
  File "/home/aolse/oils/oil/core/main_loop.py", line 280, in Interactive
    is_return, _ = cmd_ev.ExecuteAndCatch(node, 0)
  File "/home/aolse/oils/oil/osh/cmd_eval.py", line 2366, in ExecuteAndCatch
    status = self._Execute(node)
  File "/home/aolse/oils/oil/osh/cmd_eval.py", line 2164, in _Execute
    status = self._Dispatch(node, cmd_st)
  File "/home/aolse/oils/oil/osh/cmd_eval.py", line 1975, in _Dispatch
    status = self._DoExpr(node)
  File "/home/aolse/oils/oil/osh/cmd_eval.py", line 1291, in _DoExpr
    ui.PrettyPrintValue('', val, mylib.Stdout())
  File "/home/aolse/oils/oil/display/ui.py", line 582, in PrettyPrintValue
    if TypeNotPrinted(val):
  File "/home/aolse/oils/oil/display/ui.py", line 556, in TypeNotPrinted
    return val.tag() in (value_e.Null, value_e.Bool, value_e.Int,
AttributeError: 'NoneType' object has no attribute 'tag'

The root cause is that we return None in func_hay.ParseHay on a parse error:

oils/builtin/func_hay.py

Lines 66 to 71 in 16ef54b

	try:
	with alloc.ctx_SourceCode(arena, src):
	node = main_loop.ParseWholeFile(c_parser)
	except error.Parse as e:
	self.errfmt.PrettyPrintError(e)
	return None

There are two possible fixes:

1. We return value.Null,
2. We raise an error.Expr from parseHay

I think (2) is best.