(core) Finish fixing default cell color: error cells should also use black color.

Summary:
Previous diff (https://phab.getgrist.com/D2736) broke a test, which I assumed
was unrelated, but should have checked.

In fact, a few tests were changed to pass with the changed default color
(although no such change was planned). This diff reverts those changes, and
reverts also the change to error cells, so that their text is also black again.

Test Plan: Checked affected tests.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D2737

app/client/components/viewCommon.css

@@ -62,7 +62,7 @@
 
 .field_clip.invalid {
   background-color: #ffb6c1;
-  color: unset;
+  color: black;
 }
 
 .field_clip.invalid:empty {