Catch CLI errors from underlying commands and make them pretty.

This:

``` bash
$ nsot-server start
Performing upgrade before service startup...
Performing collectstatic before service startup...
Traceback (most recent call last):
File "/tmp/nsot/envs/nsot/bin/nsot-server", line 8, in
load_entry_point('nsot==0.8.5', 'console_scripts', 'nsot-server')()
File "/tmp/nsot/envs/nsot/lib/python2.7/site-packages/nsot/util.py", line 192, in main
initializer=initialize_app,
File "/tmp/nsot/envs/nsot/lib/python2.7/site-packages/logan/runner.py", line 193, in run_app
management.execute_from_command_line([runner_name, command] + command_args)
File "/tmp/nsot/envs/nsot/lib/python2.7/site-packages/django/core/management/__init__.py", line 338, in execute_from_command_line
utility.execute()
File "/tmp/nsot/envs/nsot/lib/python2.7/site-packages/django/core/management/__init__.py", line 330, in execute
self.fetch_command(subcommand).run_from_argv(self.argv)
File "/tmp/nsot/envs/nsot/lib/python2.7/site-packages/django/core/management/base.py", line 393, in run_from_argv
self.execute(*args, **cmd_options)
File "/tmp/nsot/envs/nsot/lib/python2.7/site-packages/raven/contrib/django/management/__init__.py", line 41, in new_execute
return original_func(self, *args, **kwargs)
File "/tmp/nsot/envs/nsot/lib/python2.7/site-packages/django/core/management/base.py", line 444, in execute
output = self.handle(*args, **options)
File "/tmp/nsot/envs/nsot/lib/python2.7/site-packages/nsot/management/commands/start.py", line 77, in handle
call_command('collectstatic', interactive=False, ignore=['src'])
File "/tmp/nsot/envs/nsot/lib/python2.7/site-packages/django/core/management/__init__.py", line 120, in call_command
return command.execute(*args, **defaults)
File "/tmp/nsot/envs/nsot/lib/python2.7/site-packages/raven/contrib/django/management/__init__.py", line 41, in new_execute
return original_func(self, *args, **kwargs)
File "/tmp/nsot/envs/nsot/lib/python2.7/site-packages/django/core/management/base.py", line 444, in execute
output = self.handle(*args, **options)
File "/tmp/nsot/envs/nsot/lib/python2.7/site-packages/django/contrib/staticfiles/management/commands/collectstatic.py", line 168, in handle
collected = self.collect()
File "/tmp/nsot/envs/nsot/lib/python2.7/site-packages/django/contrib/staticfiles/management/commands/collectstatic.py", line 107, in collect
handler(path, prefixed_path, storage)
File "/tmp/nsot/envs/nsot/lib/python2.7/site-packages/django/contrib/staticfiles/management/commands/collectstatic.py", line 305, in copy_file
if not self.delete_file(path, prefixed_path, source_storage):
File "/tmp/nsot/envs/nsot/lib/python2.7/site-packages/django/contrib/staticfiles/management/commands/collectstatic.py", line 255, in delete_file
self.storage.delete(prefixed_path)
File "/tmp/nsot/envs/nsot/lib/python2.7/site-packages/django/core/files/storage.py", line 289, in delete
os.remove(name)
OSError: [Errno 13] Permission denied: '/etc/nsot/staticfiles/build/js/app.min.js'
```

Should print out pretty instead.