server: print actual model name in 'model not found" error (#19117)

server: print actual model name in 'model not found" error (#19117)

Experimenting with AI, my environment gets messy fast and it's not
always easy to know what model my software is trying to load. This helps
with troubleshooting.

before:

Error: {
  code = 400,
  message = "model not found",
  type = "invalid_request_error"
}

After:

Error: {
  code = 400,
  message = "model 'toto' not found",
  type = "invalid_request_error"
}