Change classes and class handling for clear all and clear error buttons

index.html

@@ -11,8 +11,8 @@
     <main>
         <div class="calculator">
             <div class="screen">0</div>
-            <button class="clear">C</button>
-            <button class="back">CE</button>
+            <button class="clear all">C</button>
+            <button class="clear error">CE</button>
             <button class="num">7</button>
             <button class="num">8</button>
             <button class="num">9</button>