fix(helm): handle errors when plugin command is not found

If a 'command:' is not found for a plugin, it will not result in an
ExitError, but in a PathError. This prevents that condition from
panicing.

Closes #1609

cmd/helm/plugins.go

@@ -78,10 +78,12 @@ func loadPlugins(baseCmd *cobra.Command, home helmpath.Home, out io.Writer) {
 				prog.Stdout = out
 				prog.Stderr = os.Stderr
 				if err := prog.Run(); err != nil {
-					eerr := err.(*exec.ExitError)
+					if eerr, ok := err.(*exec.ExitError); ok {
 						os.Stderr.Write(eerr.Stderr)
 						return fmt.Errorf("plugin %q exited with error", md.Name)
 					}
+					return err
+				}
 				return nil
 			},
 			// This passes all the flags to the subcommand.