adding util functions
test: adding EXPECT_THROW_MSG macro
Merge branch 'develop' into feature/issue-1066-check_nonzero_size-message
test: fixing EXPECT_THROW_MSG macro
test: added a util and added EXPECT_THROW_MSG macro
fixes #1066. addresses incorrect message for check_nonzero_size
added additional doc; changed message to be clearer
Merge pull request #1068 from stan-dev/feature/issue-1066-check_nonzero_size-message

Fixes #1066. Feature/issue 1066 check nonzero size message