Reduce exceptions verbosity

All created exception already log its message. There is no need to log it beforehand.
Exemple:
```python
import logging
logging.basicConfig(level=logging.DEBUG)

try:
    season[42]
    # ERROR:pytvdbapi.api:Episode 42 not found
    # ERROR:pytvdbapi.error:Index 42 not found
except:
    pass

try:
    episode.foooo
    # ERROR:pytvdbapi.api:Episode has no attribute foooo
    # ERROR:pytvdbapi.error:Episode has no attribute foooo
except:
    pass

print hasattr(episode, 'foooo')
# ERROR:pytvdbapi.api:Episode has no attribute foooo
# ERROR:pytvdbapi.error:Episode has no attribute foooo
```