[REM] tools: remove error handling in playground

There was some code to attempt to display the error in the right
pane, whenever it occured in the app early phase.

However, it made it much harder to properly handle all cases. It could
silently swallow some errors (if more than one was done), And it was
not behaving the same way in Firefox and in Chrome.

tools/playground/templates.xml

@@ -51,10 +51,6 @@
             </p>
           </div>
         </div>
-        <div t-if="state.error" class="error">
-          <h3>Error</h3>
-          <pre t-esc="state.error"/>
-        </div>
         <div class="content" t-ref="content"/>
       </div>
   </div>