Use less return_val_if_fail checks

In C, we often check preconditions in our functions using g_return_if_fail() or g_return_val_fail(), which are macros that expand to the condition they're checking for so the logs clearly show what might be going wrong.

Vala also exposes these functions, but given that Vala compiles to a lot of jibberish temp variables, the output is not quite vague. Try to use the Vala equivalent requires to do something similar (unfortunately, we can't use this everywhere as it can only be used at the beginning of a mehod).