Bugs with additional plots

I've noticed a few issues with the adding existing plots functionality.

If the plot doesn't exist I get this error instead of the warning.

Traceback (most recent call last):
  File "/home/colm.talbot/.conda/envs/o3-population/bin/summarypages", line 8, in <module>
    sys.exit(main())
  File "/home/colm.talbot/.conda/envs/o3-population/lib/python3.7/site-packages/pesummary/cli/summarypages.py", line 208, in main
    args = func["input"](opts)
  File "/home/colm.talbot/.conda/envs/o3-population/lib/python3.7/site-packages/pesummary/core/inputs.py", line 1725, in __init__
    self.existing_plot = self.opts.existing_plot
  File "/home/colm.talbot/.conda/envs/o3-population/lib/python3.7/site-packages/pesummary/core/inputs.py", line 988, in existing_plot
    logger.warn(_does_not_exist.format(_subplot))
UnboundLocalError: local variable '_subplot' referenced before assignment

I also got an error that I was missing PIL, the solution was to install pillow, maybe this should be added to the requirements.

ModuleNotFoundError: No module named 'PIL'