multishard_mutation_query: reset failed readers to inexistent state
multishard_mutation_query: reset failed readers to inexistent state

When attempting to dismantling readers, some of the to-be-dismantled
readers might be in a failed state. The code waiting on the reader to
stop is expecting failures, however it didn't do anything besides
logging the failure and bumping a counter. Code in the lower layers did
not know how to deal with a failed reader and would trigger
`std::bad_variant_access` when trying to process (save or cleanup) it.
To prevent this, reset the state of failed readers to `inexistent_state`
so code in the lower layers doesn't attempt to further process them.

scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,avikivity/scylla