Misuse of IM_ASSERT_USER_ERROR

`IM_ASSERT_USER_ERROR()` was designed to specifically cover errors that are safely recoverable.
So e.g.
```
IM_ASSERT_USER_ERROR(gp.CurrentPlot != nullptr && !gp.CurrentPlot->SetupLocked,
"Setup needs to be called after BeginPlot and before any setup locking functions (e.g. PlotX)!"); // get plot and axis
ImPlotPlot& plot = *gp.CurrentPlot;
```
The `gp.CurrentPlot != nullptr` part of it is incorrect.

I suggest not to take action right now as I am in the process of reworking this system to provide new error handling mechanisms.
I will try to post some update here eventually, but felt it was safer to leave the info down here.