The following phrase is hard to understand - the user will not know what a "stability" is:

'CLI_ERROR_INVALID_STABILITY' => '"%s" is not a valid stability.',

I recommend changing it to:

'CLI_ERROR_INVALID_STABILITY' => '"%s" needs to be set to "stable" or "unstable".',

With a comment to translators whether "stable" and "unstable" are localizeable or not.