When a Docker agent tries to register with a token that's already bound
to another agent, the error was logged generically as "Failed to send
docker report". Users had to dig into logs to understand the issue.
Now logs a prominent error message:
"DOCKER REGISTRATION FAILED: This API token is already used by another
Docker agent. Each Docker host requires its own unique token. Generate
a new token in Pulse Settings > Agents and reinstall with the new token."
Related to #1027
fd7e80ae17