Add error message

Signed-off-by: Syrone Wong <wong.syrone@gmail.com>

shadowsocks-csharp/Encryption/MbedTLSEncryptor.cs

@@ -72,7 +72,7 @@ namespace Shadowsocks.Encryption
             }
             MbedTLS.cipher_init(ctx);
             if (MbedTLS.cipher_setup( ctx, MbedTLS.cipher_info_from_string( _cipherMbedName ) ) != 0 )
-                throw new Exception();
+                throw new Exception("Cannot initialize mbed TLS cipher context");
             /*
              * MbedTLS takes key length by bit
              * cipher_setkey() will set the correct key schedule
@@ -85,11 +85,11 @@ namespace Shadowsocks.Encryption
              *  
              */
             if (MbedTLS.cipher_setkey(ctx, realkey, keyLen * 8, isCipher ? MbedTLS.MBEDTLS_ENCRYPT : MbedTLS.MBEDTLS_DECRYPT) != 0 )
-                throw new Exception();
+                throw new Exception("Cannot set mbed TLS cipher key");
             if (MbedTLS.cipher_set_iv(ctx, iv, ivLen) != 0)
-                throw new Exception();
+                throw new Exception("Cannot set mbed TLS cipher IV");
             if (MbedTLS.cipher_reset(ctx) != 0)
-                throw new Exception();
+                throw new Exception("Cannot finalize mbed TLS cipher context");
         }
 
         protected override void cipherUpdate(bool isCipher, int length, byte[] buf, byte[] outbuf)
@@ -109,7 +109,7 @@ namespace Shadowsocks.Encryption
                 ctx = _decryptCtx;
             }
             if (MbedTLS.cipher_update(ctx, buf, length, outbuf, ref length) != 0 )
-                throw new Exception();
+                throw new Exception("Cannot update mbed TLS cipher context");
         }
 
         #region IDisposable