Handle exceptions in _get_virtual() and in _get_virtual() consumers

[eliasp]

What does this PR do?

It makes the execution of _get_virtual() in modules.aptpkg a bit more deterministic by handling SystemError/CommandExecutionError exceptions and thereby shortens this backtrace:

          ID: git-binary-icons
    Function: pkg.installed
        Name: git
      Result: False
     Comment: An exception occurred in this state: Traceback (most recent call last):
                File "/usr/lib/python2.7/dist-packages/salt/state.py", line 1703, in call                                            
                  **cdata['kwargs'])                                                                                                 
                File "/usr/lib/python2.7/dist-packages/salt/loader.py", line 1649, in wrapper                                        
                  return f(*args, **kwargs)                                                                                          
                File "/usr/lib/python2.7/dist-packages/salt/states/pkg.py", line 1062, in installed                                  
                  **kwargs)                                                                                                          
                File "/usr/lib/python2.7/dist-packages/salt/states/pkg.py", line 300, in _find_install_targets                       
                  cur_pkgs = __salt__['pkg.list_pkgs'](versions_as_list=True, **kwargs)                                              
                File "/usr/lib/python2.7/dist-packages/salt/modules/aptpkg.py", line 1182, in list_pkgs                              
                  virtpkgs_all = _get_virtual()                                                                                      
                File "/usr/lib/python2.7/dist-packages/salt/modules/aptpkg.py", line 174, in _get_virtual                            
                  apt_cache = apt.cache.Cache()                                                                                      
                File "/usr/lib/python2.7/dist-packages/apt/cache.py", line 113, in __init__                                          
                  self.open(progress)                                                                                                
                File "/usr/lib/python2.7/dist-packages/apt/cache.py", line 164, in open                                              
                  self._cache = apt_pkg.Cache(progress)                                                                              
              SystemError: E:Malformed line 2 in source list /etc/apt/sources.list (type), E:The list of sources could not be read.  
     Started: 19:37:06.154266
    Duration: 24.353 ms
     Changes: 

at least by a few lines to this one:

          ID: git-binary-icons
    Function: pkg.installed
        Name: git
      Result: False
     Comment: An exception occurred in this state: Traceback (most recent call last):
                File "/usr/lib/python2.7/dist-packages/salt/state.py", line 1703, in call                                            
                  **cdata['kwargs'])                                                                                                 
                File "/usr/lib/python2.7/dist-packages/salt/loader.py", line 1649, in wrapper                                        
                  return f(*args, **kwargs)                                                                                          
                File "/usr/lib/python2.7/dist-packages/salt/states/pkg.py", line 1062, in installed                                  
                  **kwargs)                                                                                                          
                File "/usr/lib/python2.7/dist-packages/salt/states/pkg.py", line 300, in _find_install_targets                       
                  cur_pkgs = __salt__['pkg.list_pkgs'](versions_as_list=True, **kwargs)                                              
                File "/var/cache/salt/minion/extmods/modules/aptpkg.py", line 1193, in list_pkgs                                     
                  raise CommandExecutionError(cee)                                                                                   
              CommandExecutionError: Failed to get virtual package information (E:Malformed line 2 in source list /etc/apt/sources.list (type), E:The list of sources could not be read.)                                                                                 
     Started: 20:01:10.665930
    Duration: 30.483 ms
     Changes:

What issues does this PR fix or reference?

None

Tests written?

No

[cachedout]

Thanks, @eliasp