patch 8.2.1953: Vim9: extra "unknown" error after other error

Problem:    Vim9: extra "unknown" error after other error.
Solution:   Restore did_emsg count after EXEC instruction. (closes #7254)
            Improve error message from assert_fails()

src/testdir/test_assert.vim

@@ -6,11 +6,11 @@ func Test_assert_false()
   call assert_equal(0, v:false->assert_false())
 
   call assert_equal(1, assert_false(123))
-  call assert_match("Expected False but got 123", v:errors[0])
+  call assert_match("Expected 'False' but got 123", v:errors[0])
   call remove(v:errors, 0)
 
   call assert_equal(1, 123->assert_false())
-  call assert_match("Expected False but got 123", v:errors[0])
+  call assert_match("Expected 'False' but got 123", v:errors[0])
   call remove(v:errors, 0)
 endfunc
 
@@ -21,11 +21,11 @@ func Test_assert_true()
   call assert_equal(0, v:true->assert_true())
 
   call assert_equal(1, assert_true(0))
-  call assert_match("Expected True but got 0", v:errors[0])
+  call assert_match("Expected 'True' but got 0", v:errors[0])
   call remove(v:errors, 0)
 
   call assert_equal(1, 0->assert_true())
-  call assert_match("Expected True but got 0", v:errors[0])
+  call assert_match("Expected 'True' but got 0", v:errors[0])
   call remove(v:errors, 0)
 endfunc
 
@@ -234,11 +234,11 @@ func Test_assert_fail_fails()
   call remove(v:errors, 0)
 
   call assert_equal(1, assert_fails('xxx', ['E9876']))
-  call assert_match("Expected \\['E9876'\\] but got 'E492:", v:errors[0])
+  call assert_match("Expected 'E9876' but got 'E492:", v:errors[0])
   call remove(v:errors, 0)
 
   call assert_equal(1, assert_fails('xxx', ['E492:', 'E9876']))
-  call assert_match("Expected \\['E492:', 'E9876'\\] but got 'E492:", v:errors[0])
+  call assert_match("Expected 'E9876' but got 'E492:", v:errors[0])
   call remove(v:errors, 0)
 
   call assert_equal(1, assert_fails('echo', '', 'echo command'))

src/testdir/test_vim9_script.vim

@@ -2859,6 +2859,28 @@ def Test_catch_exception_in_callback()
   unlet g:caught
 enddef
 
+def Test_no_unknown_error_after_error()
+  if !has('unix') || !has('job')
+    throw 'Skipped: not unix of missing +job feature'
+  endif
+  var lines =<< trim END
+      vim9script
+      var source: list<number>
+      def Out_cb(...l: any)
+          eval [][0]
+      enddef
+      def Exit_cb(...l: any)
+          sleep 1m
+          source += l
+      enddef
+      var myjob = job_start('echo burp', #{out_cb: Out_cb, exit_cb: Exit_cb, mode: 'raw'})
+      sleep 100m
+  END
+  writefile(lines, 'Xdef')
+  assert_fails('so Xdef', ['E684:', 'E1012:'])
+  delete('Xdef')
+enddef
+
 def Test_put_with_linebreak()
   new
   var lines =<< trim END

src/testing.c

@@ -220,7 +220,11 @@ fill_assert_error(
 	vim_free(tofree);
     }
     else
+    {
+	ga_concat(gap, (char_u *)"'");
 	ga_concat_shorten_esc(gap, exp_str);
+	ga_concat(gap, (char_u *)"'");
+    }
     if (atype != ASSERT_NOTEQUAL)
     {
 	if (atype == ASSERT_MATCH)
@@ -571,6 +575,7 @@ f_assert_fails(typval_T *argvars, typval_T *rettv)
     {
 	char_u	buf[NUMBUFLEN];
 	char_u	*expected;
+	char_u	*expected_str = NULL;
 	int	error_found = FALSE;
 	int	error_found_index = 1;
 	char_u	*actual = emsg_assert_fails_msg == NULL ? (char_u *)"[unknown]"
@@ -598,6 +603,7 @@ f_assert_fails(typval_T *argvars, typval_T *rettv)
 	    if (!pattern_match(expected, actual, FALSE))
 	    {
 		error_found = TRUE;
+		expected_str = expected;
 	    }
 	    else if (list->lv_len == 2)
 	    {
@@ -605,7 +611,10 @@ f_assert_fails(typval_T *argvars, typval_T *rettv)
 		actual = get_vim_var_str(VV_ERRMSG);
 		expected = tv_get_string_buf_chk(tv, buf);
 		if (!pattern_match(expected, actual, FALSE))
+		{
 		    error_found = TRUE;
+		    expected_str = expected;
+		}
 	    }
 	}
 	else
@@ -665,7 +674,7 @@ f_assert_fails(typval_T *argvars, typval_T *rettv)
 		actual_tv.v_type = VAR_STRING;
 		actual_tv.vval.v_string = actual;
 	    }
-	    fill_assert_error(&ga, &argvars[2], NULL,
+	    fill_assert_error(&ga, &argvars[2], expected_str,
 			&argvars[error_found_index], &actual_tv, ASSERT_OTHER);
 	    ga_concat(&ga, (char_u *)": ");
 	    assert_append_cmd_or_arg(&ga, argvars, cmd);

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1953,
 /**/
     1952,
 /**/

src/vim9execute.c

@@ -1071,8 +1071,15 @@ call_def_function(
 	{
 	    // execute Ex command line
 	    case ISN_EXEC:
+		{
+		    int save_did_emsg = did_emsg;
+
 		    SOURCING_LNUM = iptr->isn_lnum;
 		    do_cmdline_cmd(iptr->isn_arg.string);
+		    // do_cmdline_cmd() will reset did_emsg, but we want to
+		    // keep track of the count to compare with did_emsg_before.
+		    did_emsg += save_did_emsg;
+		}
 		break;
 
 	    // execute Ex command from pieces on the stack