For 2D gene plotting, fix the case when a gene has 0 expression in all samples in one region

[sjessa]

Ideally we could display an error, but otherwise we could display the plots with all 0 values, as we do in the case where only one sample has 0 expression across all cells?

In this case, the error shows up for forebrain, but not for pons:

[bhavyaac]

I plan to display a descriptive error message using validate() -"This gene has no detected expression in the selected brain region". I will display it in place of the expression-related dimensionality reduction plots & violin plots (Single cell expression by region & by sample tabs) as well as in the timecourse plot (which also displays an error in this case - see below).

[bhavyaac]

Previews for dimensionality reduction plots and violin plots:

[sjessa]

@bhavyaac I think that looks great! That validate() function looks like it will be super handy!

[bhavyaac]

Update: tested this in the server and it doesn't work currently, I'll have to do some more adjustments & try to test locally to get it working!