Gracefully handle missing files

[bpurinton]

For the final disparity, match points, etc. plots, currently an error is raised if e.g. the DEM cannot be found. This should not be the case and instead the plots that depend on the DEM should simply not be shown and no error should crash the other independent plots:

Don't do this:

asp_plot/asp_plot/stereo.py

Lines 69 to 70 in fc48806

	except:
	raise ValueError("Could not find DEM file in stereo directory")

(cc @ShashankBice)

[bpurinton]

More generally, add intelligent handling of any missing files. Scripts and plots should fail gracefully for a User. Perhaps output a plot but just show text on it that says "could not find X file(s)" along with perhaps helpful message depending on the file type of how to generate it.