Fix crash fetching cover art for unknown album

Ripping an unknown album when cover art fetching is enabled (e.g.
`whipper cd rip --unknown --cover-art complete`) causes whipper to crash
with an error similar to the following:

```python
Traceback (most recent call last):
    File "<string>", line 1, in <module>
    File ".../whipper/whipper/command/main.py", line 43, in main
    ret = cmd.do()
    File ".../whipper/whipper/command/basecommand.py", line 139, in do
    return self.cmd.do()
    File ".../whipper/whipper/command/basecommand.py", line 139, in do
    return self.cmd.do()
    File ".../whipper/whipper/command/cd.py", line 191, in do
    self.doCommand()
    File ".../whipper/whipper/command/cd.py", line 363, in doCommand
    self.program.metadata.mbid)
AttributeError: 'NoneType' object has no attribute 'mbid'
```

due to accessing `self.program.metadata.mbid` when
`self.program.metadata` is `None`. To avoid this, only attempt to get
cover art when `self.program.metadata` is available.

Also print a warning when the cover art can't be fetched to inform the
user that it isn't being downloaded.

Signed-off-by: Kevin Locke <kevin@kevinlocke.name>

whipper/command/cd.py

@@ -358,9 +358,14 @@ Log files will log the path to tracks relative to this directory.
                            "because the 'pillow' module isn't available",
                            self.options.fetch_cover_art)
         elif self.options.fetch_cover_art in {"file", "embed", "complete"}:
-            self.coverArtPath = self.program.getCoverArt(
-                                    dirname,
-                                    self.program.metadata.mbid)
+            if getattr(self.program.metadata, "mbid", None) is not None:
+                self.coverArtPath = self.program.getCoverArt(
+                                        dirname,
+                                        self.program.metadata.mbid)
+            else:
+                logger.warning("the cover art option '%s' won't be honored "
+                               "because disc metadata isn't available",
+                               self.options.fetch_cover_art)
         if self.options.fetch_cover_art == "file":
             self.coverArtPath = None  # NOTE: avoid image embedding (hacky)