service: print more error message

Some error messages were swallowed which makes user confused, for
example, for RAFSv6, we need to set blobcache config in `localfs.json`
(following docs tutorial), before modification, the error message
indicates nothing:

```
ERROR [src/bin/nydusd/main.rs:525] Failed in starting daemon: Invalid
argument (os error 22)
```

After this modification, we get clearer error message:

```
ERROR [/src/fusedev.rs:595] service mount error: RAFS failed to handle
request, Configure("Rafs v6 must have local blobcache configured")
```

Signed-off-by: Wei Zhang <weizhang555.zw@gmail.com>

docs/tutorial.md

@@ -287,6 +287,12 @@ $ cat localfs.json
       "config": {
         "dir": "/<YOUR-WORK-PATH>/nydus-image/blobs"
       }
+    },
+    "cache": {
+      "type": "blobcache",
+      "config": {
+        "work_dir": "/var/lib/nydus/cache"
+      }
     }
   },
   "mode": "direct"

service/src/fusedev.rs

@@ -591,15 +591,21 @@ pub fn create_fuse_daemon(
         || api_sock.is_none()
     {
         if let Some(cmd) = mount_cmd {
-            daemon.service.mount(cmd)?;
+            daemon.service.mount(cmd).map_err(|e| {
+                error!("service mount error: {}", &e);
+                eother!(e)
+            })?;
         }
         daemon
             .service
             .session
             .lock()
             .unwrap()
             .mount()
-            .map_err(|e| eother!(e))?;
+            .map_err(|e| {
+                error!("service session mount error: {}", &e);
+                eother!(e)
+            })?;
         daemon
             .on_event(DaemonStateMachineInput::Mount)
             .map_err(|e| eother!(e))?;