Emit informative error for non-existent attribute

[cgiachalis]

It'd better if tiledb_array() could throw an informative message when querying for non-existent attribute.

library(tiledb)
uri <- tempfile()

# Create schema only
dat <- data.frame(id = "a", val = "John")
fromDataFrame(dat, uri, col_index = 1)

arr <- tiledb_array(uri, attrs = "val-no")

arr[]
#> Error in `if (is.na(varnum) && !nullable) ...`:
#> ! missing value where TRUE/FALSE needed