create exception in plotting for case when all doses are zero for a chemical

[kdionisio]

Bug discovered in #139, see ticket for more detail. When attempting to plot 'by product' and a chemical within that product run has all zeroes for dose, the CDF calculated results in negative values, and thus the plot does not appear for any of the chemicals in that product category.

Need to include an exception in code prior to plotting which removes any chemicals within a product run which have all zeroes for dose, and only attempts to plot chemicals which have non-zero dose data, so that we do not end up with a blank plot.

[kdionisio]

Also create an exception so when chemicals are not plotting due to all zero dose for a 'by chemical' run, a message appears, such as 'all zero doses for systemic dose' etc