plugins.facebook: Add 'Log into Facebook' error message.
plugins.facebook: Add 'Log into Facebook' error message.

```
$ streamlink https://www.facebook.com/FranceRugby/posts/3017597365007921
[cli][info] Found matching plugin facebook for URL https://www.facebook.com/FranceRugby/posts/3017597365007921
[plugins.facebook][error] Video is not available, You must log in to continue.
error: No playable streams found on this URL: https://www.facebook.com/FranceRugby/posts/3017597365007921
```

melmorabity/streamlink,gravyboat/streamlink,bastimeyer/streamlink,chhe/streamlink,melmorabity/streamlink,streamlink/streamlink,streamlink/streamlink,gravyboat/streamlink,chhe/streamlink,bastimeyer/streamlink