Use a tooltip to explain whitespace errors

Sometime users are confused about why Gerrit shows a particular part
of the new file (which is always green) in red.  Its a whitespace
error, a case that isn't normal.  Explain it with a tooltip message.

Change-Id: Ide90f14c4a25eac5a2bcb1bc9f5e6d67e9ac4cb7
Signed-off-by: Shawn O. Pearce <sop@google.com>

gerrit-prettify/src/main/resources/com/google/gerrit/prettify/client/gerrit.css

@@ -21,6 +21,7 @@
 
 .wse {
   background: red;
+  cursor: pointer;
 }
 
 .vt {