Throw in AES-CBC decrypt operation for odd ciphertext lengths (#382)

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

Author: twiss

spec/Overview.html

@@ -12710,6 +12710,12 @@ <h5>Decrypt</h5>
                   {{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