add missing thumbnail check to verification

[lsat12357]

at the moment, the missing thumbnails are actually only missing from solr, can fix by reindexing.
strictly speaking, this is not a migration fail, but meh, close enough.
this should be a task that queries to make sure that the thumbnail field is populated in solr, and if not, initiates reindex

[lsat12357]

so afaik this is one line of code to check, and then one more line to fix.
since accessing the solr doc is available via HyraxCore, something like
Hyrax::Migrator::HyraxCore::Asset.solr_record(asset.pid)['thumbnail_path_ss'].blank?
and then to set the thumbnail path
asset.update_index
seems overkill to add this to the verification process, also upon reflection, verification should not be fixing stuff.

[lsat12357]

could add this to the derivatives check.