Do not throw when CAST5 unsupported

While this message may be more helpful, it will break some cases, such
as when there are multiple ciphers that could be used and we can just
skip CAST5 and move on.  Return NULL when CAST5 unsupported, just like
for other unsupported ciphers.

lib/openpgp_crypt_symmetric.php

@@ -159,8 +159,6 @@ class OpenPGP_Crypt_Symmetric {
           $cipher = new OpenSSLWrapper("CAST5-CFB");
         } else if(defined('MCRYPT_CAST_128')) {
           $cipher = new MCryptWrapper(MCRYPT_CAST_128);
-        } else {
-          throw new Exception("Unsupported cipher: you must have mcrypt installed to use CAST5");
         }
         break;
       case 4: