more informative error reporting

[saravkin]

Some users may do unexpected things, like have NA values in SDs. In this case, we should error out in an informative way, and tell them that they cannot have NAs (or e.g. <= 0 values). Currently the resulting error is hard for them to interpret:

[dir]/miniconda3/envs/mrtool_0.0.1/lib/python3.7/site-packages/limetr/init.py:70: RuntimeWarning: invalid value encountered in square
self.V = S**2
Error in py_call_impl(callable, dots$args, dots$keywords) :
AssertionError