Handle aggregation input that cannot be split without dying

Improperly formatted aggregation dump data can cause whisper-aggregate to die on a ValueError thrown by split().

```
Traceback (most recent call last):
  File "/usr/bin/whisper-aggregate", line 9, in <module>
    load_entry_point('carbonate==0.2.1', 'console_scripts', 'whisper-aggregate')()
  File "/usr/lib/python2.7/dist-packages/carbonate/cli.py", line 321, in whisper_aggregate
    name, t = metric.strip().split('|')
ValueError: need more than 1 value to unpack
```

This wrap the split call in a try and reports a warning instead of failing the aggregation run.