Distinguish "repository doesn't exist" from "repository plugin not installed"

[DaveCTurner]

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.

[elasticmachine]

Pinging @elastic/es-distributed (Team:Distributed)