Distinguish "repository doesn't exist" from "repository plugin not installed" #81758
Description
Elasticsearch version (bin/elasticsearch --version): 7.16.1 (and many others)
Plugins installed: []
JVM version (java -version): Any
OS version (uname -a if on a Unix-like system): Any
Description of the problem including expected versus actual behavior:
If you define a repository using a plugin and then remove the plugin then the RepositoryMetadata lives on but we do not create a corresponding Repository instance. This means that operations on the repository (creating snapshots etc) will fail with a RepositoryMissingException which is very confusing because GET _snapshot will show that the repository is not missing at all.
I believe in this case we should create a placeholder Repository instance so that operations on the repository fail with a more helpful message.