Implement propper error handling in CLI client

[forman]

Is your feature request related to a problem? Please describe.

The CLI client has no special error treatment for errors raised when attempting to reach the server or if server API calls failed. It will always dump the traceback as if it were internal program errors.

Describe the solution you'd like

We already raise

• TransportException when attempting to reach the server
• ClientException if server API request failed

--> Catch these errors in the CLI and output human-readable reports. Provide traceback only if CLI flag is set.

[forman]

Addressed in #55