catlog traceback when no units are specified

[markpeek]

@samrushing I'm sure we can catch this and give a better error message.

$ catlog -a 10 /var/log/output.log
Traceback (most recent call last):
  File "/usr/local/bin/catlog", line 4, in <module>
    __import__('pkg_resources').run_script('coro==1.0.5', 'catlog')
  File "build/bdist.linux-x86_64/egg/pkg_resources/__init__.py", line 723, in run_script
  File "build/bdist.linux-x86_64/egg/pkg_resources/__init__.py", line 1643, in run_script
  File "/usr/local/lib/python2.7/dist-packages/coro-1.0.5-py2.7-linux-x86_64.egg/EGG-INFO/scripts/catlog", line 110, in <module>

  File "/usr/local/lib/python2.7/dist-packages/coro-1.0.5-py2.7-linux-x86_64.egg/EGG-INFO/scripts/catlog", line 40, in main

  File "/usr/local/lib/python2.7/dist-packages/coro-1.0.5-py2.7-linux-x86_64.egg/EGG-INFO/scripts/catlog", line 67, in seek

AttributeError: 'NoneType' object has no attribute 'groups'