fix: print cleaner error message if authentication failed

[kangasta]

Missing credentials

$ upctl server show asd
Error: user credentials not found, these must be set in config file (used /Users/kangasta/.config/upctl.yaml) or via environment variables
$ echo $?
102

Invalid credentials

$ upctl server show asd
Error: invalid user credentials, authentication failed using the given username and password
$ echo $?
103