Standardize how we denote terms in ReQL error messages

[danielmewes]

This discussion came up in #5741 (comment) .
Some of our error messages use all caps for ReQL terms (e.g. ORDER_BY), and others use lower-case (order_by).
I think lower-case terms look better, and they also correspond to what the terms are called in half of our drivers which is better than none I think.

We should standardize this and change the existing error messages.