Make it easier to understand when registry is misformatted.

[jayunit100]

Bug description

If you do something minor like add qoutes to a registry on accident, you get a tricky to udnerstand error message:

[!] Unable to resolve the plugin discovery image: unable to fetch registry name from image "'projects.registry.vmware.com/tanzu_cli_stage/plugins/plugin-inventory:latest'": could not parse reference: 'projects.registry.vmware.com/tanzu_cli_stage/plugins/plugin-inventory:latest'
[!] unable to list groups from discovery 'disc_0': unable to fetch the inventory of discovery 'disc_0' for groups: plugins discovery image resolution failed. Please check that the repository image URL "'projects.registry.vmware.com/tanzu_cli_stage/plugins/plugin-inventory:latest'" is correct
[x] : unable to find plugin group vmware-tkg/tkg-ob-464796462228756226-g885cc5ad

Expected behavior

We should just bail out if the registry string is clearly invalid, b/c "parse reference" isnt clear that its a simple "registry has weird chars in it" related error.

Steps to reproduce the bug / Relevant debug output

Output of tanzu version

Environment where the bug was observed (cloud, OS, etc)