Do not throw if the secrets manager is disabled

[brichet]

To be able to use the secrets manager, the plugin need to sign before being activated. But we could still expect the extension to be activated without it.
The sign function throw if the secrets manager is disabled, which prevents the plugins to get activated without the secrets manager activated.

This PR only warns if the manager is disabled, leaving the plugin concerned to manage its own secrets.

cc. @afshin who was the first to implement the sign function.

[brichet]

Don't know how I closed this one.

[afshin]

This change makes sense. I would simplify the lock function to have the same signature as before but if message is an empty string, to simply not throw.

(I don't feel strongly about this, though, and either way your change is reasonable.)

[brichet]

Thanks for the suggestion.
I updated the PR, but the signature changed anyway, the message argument is now optional. The error is thrown only if message is provided.