Print error message if no rt server was found

[talarian1]

Is your feature request related to a problem? Please describe.
Today, if no secret with the desired prefix is found, no informative message is printed.
Which will probably led to the following message to be printed by the JFrog CLI commands:

Error: the --url option is mandatory

Describe the solution you'd like to see
Add check for existing rt server, and print an informative error message in case no matching secret was found.