Valid command producing an error shows usage string unnecessarily when using "list variables"

[wolcen]

Describe the bug
Listing variables that results in an error, also displays the Usage text unnecessarily. This occurs for me when an environment does not have any variables as yet.

To Reproduce
Steps to reproduce the behavior:

1. Run lagoon list variables -p yourenv for an environment having no existing variables.
2. See error

Expected behavior
The error is correct; The display of command usage is unnecessary/potentially confusing.

Screenshots

$ lagoon list variables -p example-com
Error: There are no variables for project 'example-com'
Usage:
  lagoon list variables [flags]

Aliases:
  variables, v
...

Desktop (please complete the following information):

• OS: Linux

[tobybellwood]

Closed in #298