Throw in AES-CBC decrypt operation for odd ciphertext lengths

If the ciphertext is not a positive multiple of 16 bytes, return an
OperationError.

Author: twiss

spec/Overview.html

@@ -10662,6 +10662,12 @@ <h4>Operations</h4>
                     {{OperationError}}.
                   </p>
                 </li>
+                <li>
+                  <p>
+                    If the length of |ciphertext| is zero or is not a multiple
+                    of 16 bytes, then [= exception/throw =] an {{OperationError}}.
+                  </p>
+                </li>
                 <li>
                   <p>
                     Let |paddedPlaintext| be the result of performing the CBC Decryption