Handle invalid subject values within PlumJsonldConverter

[jrgriffiniii]

For cases where subject values are string literals (and are expected to be URI's to vocabulary term resources), log a warning during the indexing process.

[hackartisan]

Did this happen with real data or just in testing? I've noticed a lot of strings used in testing in figgy at least.

[escowles]

I wouldn't be surprised if there are some strings lurking in the LAE data. But even if not, I think it's good to make this code more robust and flexible, instead of depending on the LAE ephemera configuration to guarantee that we're only going to see IDs in the subject field.