JSON deserialization can fail

tell the user, not just the logs, when:
```
abusctl call org.opensuse.Agama.Storage1 /org/opensuse/Agama/Storage1 org.opensuse.Agama.Storage1 GetConfig
s "null"
```

rust/agama-lib/src/storage/client.rs

@@ -148,7 +148,7 @@ impl<'a> StorageClient<'a> {
     pub async fn set_config(&self, settings: StorageSettings) -> Result<u32, ServiceError> {
         Ok(self
             .storage_proxy
-            .set_config(serde_json::to_string(&settings).unwrap().as_str())
+            .set_config(serde_json::to_string(&settings)?.as_str())
             .await?)
     }
 
@@ -160,7 +160,7 @@ impl<'a> StorageClient<'a> {
     /// Get the storage config according to the JSON schema
     pub async fn get_config(&self) -> Result<StorageSettings, ServiceError> {
         let serialized_settings = self.storage_proxy.get_config().await?;
-        let settings = serde_json::from_str(serialized_settings.as_str()).unwrap();
+        let settings = serde_json::from_str(serialized_settings.as_str())?;
         Ok(settings)
     }