Clarify error when discovery source is invalid

[marckhouzam]

What this PR does / why we need it

Clarify error when a plugin discovery source is invalid versus when it is not yet supported.

Which issue(s) this PR fixes

Fixes #3078

Describe testing done for PR

Before the PR we can see that an invalid discovery source of type fish gives the same error as an valid but unsupported discovery source type of GCP:

$ tanzu plugin source add -n test -u example.com/test --type fish
✖  discovery source type 'fish' is not yet supported
$ tanzu plugin source add -n test -u example.com/test --type GCP
✖  discovery source type 'GCP' is not yet supported

With the PR applied, only valid discovery source types give the unsupported error, while others give an invalid error:

$ tz plugin source add -n test -u example.com/test --type fish
✖  unknown discovery source type 'fish'
$ tz plugin source add -n test -u example.com/test --type GCP
✖  discovery source type 'GCP' is not yet supported
$ tz plugin source add -n test -u example.com/test --type kubernetes
✖  discovery source type 'kubernetes' is not yet supported
$ tz plugin source add -n test -u example.com/test --type rest
✖  discovery source type 'rest' is not yet supported

Release note

Improved error messages when the user attempts to add a discovery source of an invalid type.

[anujc25]

LGTM. Thanks for the fix.

[marckhouzam]

Thanks for the quick review @anujc25 !