patch 8.2.4296: Vim9: not all code covered by tests

Problem:    Vim9: not all code covered by tests.
Solution:   Add a few more tests for corner cases.  Fix hang when single quote
            is missing.

src/testdir/test_vim9_assign.vim

@@ -1993,6 +1993,12 @@ def Test_unlet()
   assert_false(exists('s:somevar'))
   unlet! s:somevar
 
+  if 0
+    unlet g:does_not_exist
+  endif
+
+  v9.CheckDefExecFailure(['unlet v:notfound.key'], 'E1001:')
+
   v9.CheckDefExecFailure([
     'var dd = 111',
     'unlet dd',

src/testdir/test_vim9_cmd.vim

@@ -1449,7 +1449,7 @@ def Test_lockvar()
     ex = v:exception
   endtry
   assert_match('E1121:', ex)
-  unlockvar d.a
+  unlockvar d['a']
   d.a = 7
   assert_equal({a: 7, b: 5}, d)
 

src/testdir/test_vim9_expr.vim

@@ -1833,6 +1833,7 @@ def Test_expr8_string()
 
   v9.CheckDefAndScriptFailure(['var x = "abc'], 'E114:', 1)
   v9.CheckDefAndScriptFailure(["var x = 'abc"], 'E115:', 1)
+  v9.CheckDefFailure(["if 0", "echo 'xx", "endif"], 'E115', 2)
 enddef
 
 def Test_expr8_vimvar()

src/version.c

@@ -746,6 +746,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4296,
 /**/
     4295,
 /**/

src/vim9expr.c

@@ -2824,8 +2824,10 @@ compile_expr1(char_u **arg, cctx_T *cctx, ppconst_T *ppconst)
     // Ignore all kinds of errors when not producing code.
     if (cctx->ctx_skip == SKIP_YES)
     {
+	int		prev_did_emsg = did_emsg;
+
 	skip_expr_cctx(arg, cctx);
-	return OK;
+	return did_emsg == prev_did_emsg ? OK : FAIL;
     }
 
     // Evaluate the first expression.