Warn when using non HTTPS connection

[mluisbrown]

When trying to decrypt secrets on my homepage, having accessed it with http instead of https I got the rather unhelpful error:

Decryption failed: TypeError: Cannot read properties of undefined (reading 'importKey')

Only after checking your example with an incorrect password did I realise the error was not related to my password (which I was sure I had correct).

Then I realised I was using http and that might cause Crypto APIs to not work, which was indeed the problem. It might be helpful to warn users that this only works over an SSL connection.