Add info to pytest.fail msg when image not found, to indicate baseline_dir where plugin was looking

[amoodie]

Really simple change to pytest.fail msg for when the plugin fails to find the image at baseline_dir. I'm new to the plugin, and when attempting to use, I struggled for longer than I'd like to admit as to why the test could not find my image.

My baseline_dir was located in my tests folder alongside the test_figs.py test file, as the instructions indicate to do after generating the baseline figures. The instructions also say (on my re-reading now) that the folder is "interpreted as being relative to the test file," but I foolishly was defining @pytest.mark.mpl_image_compare(baseline_dir='tests/figs_baseline').

Anyway, I only figured this out by modifying my installed version of pytest-mpl to determine where the plugin was searching. I'm not sure if anyone else follows my stupidity, but this would be a simple debugging tool for new users. Also might be helpful to add some emphasis to the README in this vein? Or a simple project structure example like below?

src/
  figure_maker.py
  tests/
    test_figure_maker.py
    baseline/
      test_figure_maker_fig1.png
      test_figure_maker_fig2.png

where figure_maker.py is a script or module that produces fig1 and fig2 during normal use.

Anyway, great plugin/utility, thanks!
AM

[astrofrog]

Good idea - thanks!