Fixed TypeError

I didn't have ``pymysql`` installed but the error reporting code crashed with the following error:

```
Traceback (most recent call last):
  File "venv/bin/stellar", line 9, in <module>
    load_entry_point('stellar==0.4.1', 'console_scripts', 'stellar')()
  File "/path/venv/lib/python2.7/site-packages/stellar/command.py", line 280, in main
    library,
TypeError: not enough arguments for format string
```