improve error message

[yuhanH]

pbmc_small
pbmc_small <- NormalizeData(pbmc_small)%>%FindVariableFeatures()%>%ScaleData()%>%RunPCA()
pbmc_small <- FindNeighbors(pbmc_small, dims = 1:30, return.neighbor = T)
pbmc_small <- FindNeighbors(pbmc_small, dims = 1:30, return.neighbor = F)

It changes the error message from

Error: This object already contains RNA_nn as a Neighbor; duplicate names are not allowed

to

Error: This object already contains RNA_nn as a Neighbor, so RNA_nn cannot be used for a Graph