Replace return calls after g_error() with g_assert_not_reached()
In a number of places, there is a return call after a call to g_error(). The original intention was to stop the function just in case g_error() returns (which it shouldn't do) and also because it gives a quick impression to any naïve inspectors that the program flow won't leave the if-block. However, it would be better if g_assert_not_reached() was called instead as this is more appropriate and achieves the same goals.