From my point of view you shouldn't hide the exception at that level. Throwing an exception when an user tries to decrypt an unencrypted piece of data seems the expected behavior of a cryptography module to me.
Instead, you should deal with this exception at a higher level (outside the cryptography module). For instance, you could have something like this in your UI layer:
try
{
var encryptedFilePath = ShowOpenFileDialog();
var decryptedFilePath = TryDecryptFile(encryptedFilePath);
ShowMessagePopup("Your file has been decrypted to: " + decryptedFilePath);
}
catch (CryptographicException)
{
ShowErrorPopup("Unable to decrypt file!\n" +
"Please make sure the file you selected is valid");
}