Review http status code handling on all network operation

[DeepDiver1975]

Background

Http status codes have defined meanings and the client has to act accordingly.
E.g. 503/Service Unavailable should trigger a pause in operations until the service is back again. In case this scenario is triggered on a token refresh it should not trigger a re-login what would be reasonable in case of a 401

Tasks

• Review any network job - https://github.com/search?q=repo%3Aowncloud%2Fclient%20QNetworkReply%3A%3Afinished&type=code
• Double check the specifications in case of WebDAV, GraphAPI and OAuth/OpenIdConnect
• Double check server implementations
• List findings below

Findings

OpenID Connect Service Discovery

ref:

client/src/libsync/creds/oauth.cpp

Line 527 in 40b29d2

// Most likely the file does not exist, default to the normal endpoint

Only in case of a 404 we can assume that the server is not supporting OpenID Connect. Any other http status code - specifically 503 shall lead into dedicated error handling.
TODO: double check the specifications