Fail-fast TileDB reader

[DPeterK]

Currently if you load a TileDB array that doesn't exist on disk it doesn't fail, but instead returns an empty cubelist:

reader = TDBReader('/not/a/tiledb_array')
reader.to_iris()
...
...
[]

Fix this so that it fails fast, rather than running for a long time and returning an empty list.

[ihnorton]

@DPeterK in case it helps, you can use tiledb.object_type("<path>") to check if an array exists (returns either "array", "group", or None).

[DPeterK]

Thanks @ihnorton!