feat(repo): Improve error message for missing repositories.yaml

New users to helm don't always run `helm init` (e.g. if their cluster
already has helm installed).  The user's initial interaction with any of
helm's repository commands (e.g. `helm repo list`) will then be an error
message due to a missing repositories.yaml in their local config.

Give the user a little hint about how to fix the error without them having
to hunt through the man/help pages.

pkg/repo/repo.go

@@ -55,6 +55,13 @@ func NewRepoFile() *RepoFile {
 func LoadRepositoriesFile(path string) (*RepoFile, error) {
 	b, err := ioutil.ReadFile(path)
 	if err != nil {
+		if os.IsNotExist(err) {
+			return nil, fmt.Errorf(
+				"Couldn't load repositories file (%s).\n"+
+					"You might need to run `helm init` (or "+
+					"`helm init --client-only` if tiller is "+
+					"already installed)", path)
+		}
 		return nil, err
 	}