Input error checks added: plotSpectrum, plotMD, plotBin, re #8419
Better error messages, re #8419
New mantidplotpy test: MantidPlotInputArgsCheck, re #8419
remove reminder comment, re #8419
Merge branch 'bugfix/8419_better_input_check_for_python_plots' into develop, re #8419

Conflicts:
	Code/Mantid/MantidPlot/CMakeLists.txt

There was no apparent true conflict