Make width of error message relative.

Previously the width of the error message block was static and messages
would go outside the boundary of the border. Change the width to be
relative to make better use of the available space.

htmloutput/htmloutput.py

@@ -262,7 +262,7 @@ a.popup_link:hover {
     font-family: "Lucida Console", "Courier New", Courier, monospace;
     text-align: left;
     font-size: 8pt;
-    width: 500px;
+    width: 90%;
 }
 
 }