xds: fix xdsClient resource not exist for invalid resource, fix xdsServerWrapper start on resource not exist - #8660

…rverWrapper start on resource not exist (grpc#8660)

Fix bugs:
1. Invalid resource at xdsClient, the watcher should have been delivered an error instead of resource not found.
2. If the resource is properly determined to not exist, it shouldn't cause start() to fail. From A36 xDS for Servers:
"XdsServer's start must not fail due to transient xDS issues, like missing xDS configuration from the xDS server."

…rverWrapper start on resource not exist (1.42.x backport) (#8690)

* xds: remove filter chain uuid name generator (#8663)

Generating a uuid in filterChain breaks the de-duplication detection which causes XdsServer to cycle connections, so removing it.
An empty name is now allowed. The name is currently only used for debug purpose.

* xds: fix xdsClient resource not exist for invalid resource, fix xdsServerWrapper start on resource not exist (#8660)

Fix bugs:
1. Invalid resource at xdsClient, the watcher should have been delivered an error instead of resource not found.
2. If the resource is properly determined to not exist, it shouldn't cause start() to fail. From A36 xDS for Servers:
"XdsServer's start must not fail due to transient xDS issues, like missing xDS configuration from the xDS server."