Improve error message in decrypt template when crypto.subtle is missing

[Greenheart]

The current message only refers to using a "modern browser".

pagecrypt/web/decrypt.ts

Line 62 in 38b21af

error('Please use a modern browser.')

However, since this error also appears in insecure browser contexts (http, file), it would be more useful to have an error message stating that the crypto is unavailable.

We could check if the page is viewed in an insecure context, and adapt the message based on that. For example we could check window.isSecureContext and then link to https://developer.mozilla.org/en-US/docs/Web/Security/Secure_Contexts