changed implementation of covariance and correlation to throw exception instead of outputting to std::cerr
changed implementation of covariance and correlation to throw exception instead of outputting to std::cerr

stan-dev/math,stan-dev/math,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/stan,stan-dev/math,stan-dev/math,stan-dev/math