use posterior sds, when available, for test error thresholds (#545)
use posterior sds, when available, for test error thresholds (#545)

this fixes the currently broken galsim unit test, because one parameter (first color ratio) in the
`star_with_noise` test is barely outside of the allowed error bounds
jeff-regier/Celeste.jl,jeff-regier/Celeste.jl,jeff-regier/Celeste.jl