Add NodeMightBeDown to redis client error mapping

[learoyklinginsmith]

We've experienced RedisClient::Cluster::NodeMightBeDown errors when scaling in our cluster mode enabled Elasticache clusters. This seems expected, but this error doesn't exist in the error map, so we receive an exception similar to the following:

/var/bundle/ruby/3.1.0/gems/redis-clustering-5.0.8/lib/redis/cluster/client.rb:89:in 'fetch': key not found: RedisClient::Cluster::NodeMightBeDown
Did you mean?  RedisClient::Cluster::ErrorCollection (KeyError)
from /var/bundle/ruby/3.1.0/gems/redis-clustering-5.0.8/lib/redis/cluster/client.rb:89:in `rescue in handle_errors'
from /var/bundle/ruby/3.1.0/gems/redis-clustering-5.0.8/lib/redis/cluster/client.rb:79:in `handle_errors'
from /var/bundle/ruby/3.1.0/gems/redis-clustering-5.0.8/lib/redis/cluster/client.rb:61:in `call_v'
from /var/bundle/ruby/3.1.0/gems/redis-5.0.8/lib/redis.rb:152:in `block in send_command'
from /var/bundle/ruby/3.1.0/gems/redis-5.0.8/lib/redis.rb:151:in `synchronize'
from /var/bundle/ruby/3.1.0/gems/redis-5.0.8/lib/redis.rb:151:in `send_command'
from /var/bundle/ruby/3.1.0/gems/redis-5.0.8/lib/redis/commands/strings.rb:95:in `set'

This adds the NodeMightBeDown error to the error mapping so it can be fetched as expected.

[byroot]

Merged as c6fa9f4