[Fleet] Error when uploading a package that does not exist in the registry

[neptunian]

As part of #77986, the user should be able to upload a package that does not exist in the registry, instead an error is returned. An error is also returned if an uploaded package exists in the registry but the version does not.

To reproduce:

1. Create a zip of a package in the x-pack/test/ingest_manager_api_integration/apis/fixtures/direct_upload_packages directory called custom_0.1.4.zip. I did this by unzipping the the existing apache package in the directory, making some changes, and creating a new one.
2. Try to install by upload curl -X POST -u elastic:changeme http://localhost:5603/ssg/api/fleet/epm/packages --data-binary @$KIBANA_HOME/x-pack/test/ingest_manager_api_integration/apis/fixtures/direct_upload_packages/custom_0.1.4.zip -H 'kbn-xsrf: xyz' -H 'Content-Type: application/zip'
   Response:
   {"statusCode":502,"error":"Bad Gateway","message":"'404 Not Found' error response from package registry at https://epr-snapshot.elastic.co/package/custom/0.1.4/"}. Looking in the saved object, the package is stuck in "installing"

The above error probably comes from the index patterns issue, so fixing that first may resolve this #83311

[elasticmachine]

Pinging @elastic/ingest-management (Team:Ingest Management)

[ph]

I've put that in ready