enhancement for `print-string-secret-key-deprecation`

[trevor]

print-string-secret-key-deprecation now prints:

"WARNING: The secret key for the session cookie store should be a"
"byte array.\nString secret keys have been deprecated."

It took some time to track down where this warning was coming from. A helpful modification may be something along the lines:

"WARNING (library 'ring' <version>): The secret key for the session cookie store should be a"
"byte array.\nString secret keys have been deprecated."

[weavejester]

That's a good idea.