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>
3ec17dbd33