[r] Consider a more robust way to handle R rendering of non-string enum values

[johnkerl]

Tracking task for possible follow-up from #1835 (comment)

Namely:

• SOMA uses Arrow as its data model
• Arrow dictionary types have an index type and a value type
• The index type is nominally int8, int16, int32, int64
• The value type is quite general: usually string but also including various float/int widths, etc
• TileDB-SOMA-Py supports read and write of various value-types
• In R, factors must be string-valued
• Taken together, this means that we can write things that we cannot read back as R factors

The above-linked discussion shows what happens when we try and fail. The status quo may be good enough; or, maybe we can do a bit more to warn the user why they are seeing non-renderable results.