flake8 integration - do not open on new file

[raven42]

When opening a new python file that does not exist on the file system, and when flake8 is set to load on start, vim reports the following error on init because the file is not open.

Error detected while processing VimEnter Autocommands for "*.py"..function devpanel#DevPanelOpen[31]..flake8#Flake8[1]..
<SNR>85_Flake8[65]..<SNR>85_PlaceMarkers:
line   31:
E885: Not possible to change sign Flake8_Error
Press ENTER or type command to continue

After hitting enter, the following is shown in the flake8 window:

blah.py|| E902 FileNotFoundError: [Errno 2] No such file or directory: 'blah.py'

Need to update this to not load flake8 window for a new file