Assert statements need to be more verbose

[pmelchior]

As is the asserts, e.g. for number of constraints in GLMM, tell if something isn't right, but they leave the user guessing what has caused the problem.

Instead we should issue, at the very least, the condition that is being tested and why it isn't met. While we're at it we should replace the asserts with proper exceptions so that they cannot be optimized away.