Improve error handling if Cluster Based covariate has more clusters than available in data.

[jmelott]

To reproduce:
Create small 10x10 heat map using attached matrix
matrix_03_10_10_genes_genes.tsv.txt

Click next twice to get to "Cluster Matrix" page.
Enable advanced features.
Leave most features at default settings.
Click "Add Cluster-Based Covariate" for both axes.
Select 10 as number of clusters for both axes.
Click apply.
Error appears, but doesn't make it clear what needs to be changed
BUILD ERROR: Importing Heat Map Configuration Data. Index 10 out of bounds for length 10
In other cases error may look like
BUILD ERROR: Importing Heat Map Configuration Data. Index -1 out of bounds for length 6

Either check for size of data should disable cluster sizes that are too small, and/or data error should be more clear that the number of clusters need to be changed.