Allow modules to show warnings

[MatteoNardi]

Currently the module status is defined in pulsar-core/src/pdk/daemon.rs like this:

pub enum ModuleStatus {
    Created,           
    Running,           
    Failed(String),    
    Stopped,           
}                      

We have no way to show a warning for running modules. Failed contains an error message, but the module is not running in that state.

For example, a module which sends events to a remote server has no way to report that the server is down. If we move to the Failed state, it will prevent us from further connection attempts.

We should add an optional warning message to the Running state, this would be picked up and shown by the CLI whe running pulsar status.