Clarify error message for duplicate packages on install

Consider the scenario where the user has two registries 'incubator', and
'incubatorXYZ' pointing to the same repository.

It might be confusing why running `ks pkg install incubatorXYZ/mysql`
fails if they ran `ks pkg install incubator/mysql` previously.

This is a small change to help users resolve that scenario by passing
the `--name` flag on install.

Signed-off-by: Jessica Yuen <im.jessicayuen@gmail.com>

metadata/registry.go

@@ -151,7 +151,7 @@ func (m *manager) CacheDependency(registryName, libID, libName, libVersion strin
 	}
 
 	if _, ok := appSpec.Libraries[libName]; ok {
-		return nil, fmt.Errorf("Library '%s' already exists", libName)
+		return nil, fmt.Errorf("Package '%s' already exists. Use the --name flag to install this package with a unique identifier", libName)
 	}
 
 	// Retrieve registry manager for this specific registry.