Fix PercentageFeatureSet to respect the assay parameter

[dcollins15]

This PR makes two small changes to PercentageFeatureSet:

1. Correctly passes assay to the Layers call on line 1178 of utilities.R.
2. Raises a warning when the features parameter contains values not present in the specified assay instead of erroring out.

My intent is to introduce a proper test for PercentageFeatureSet for now, the following scripts can be used to verify the fixes. Verify that assay is being respected:

library(testthat)
library(Seurat)

counts <- LayerData(pbmc_small, assay = "RNA", layer = "counts")
data <- LayerData(pbmc_small, assay = "RNA", layer = "data")

assay_left <- CreateAssay5Object(data = data)
assay_right <- CreateAssay5Object(counts)

test_case <- CreateSeuratObject(assay_left, assay = "LEFT")
test_case[["RIGHT"]] <- assay_right

DefaultAssay(test_case) <- "RIGHT"
expected <- PercentageFeatureSet(test_case, assay = "RIGHT", pattern = "^RP[LS]")

DefaultAssay(test_case) <- "LEFT"
result <- PercentageFeatureSet(test_case, assay = "RIGHT", pattern = "^RP[LS]")

expect_true(identical(result, expected))

And check that a warning (not an error) is raised when features contains bad values:

PercentageFeatureSet(pbmc_small, features = "not-a-gene")