mixer: avoid extra output when mixer fails

RunE will show the error returned at each command in the tree, which
means most of our commands will be showing the error twice. Setting
SilenceErrors disables this behavior.

RunE will show the usage when an error happens, which most of the time
is just "spam" since the call might be correct and the error happened
during the execution of the command.

Before this patch a failure looked like

    $ mixer add-rpms
    Mixer 3.2.1
    Error: RPMDIR not set in configuration
    Usage:
      mixer add-rpms [flags]

    Flags:
      -c, --config string   Builder config to use
      -h, --help            help for add-rpms

    Mixer Error: RPMDIR not set in configuration

and after the patch it looks like

    $ mixer add-rpms
    Mixer 3.2.1
    Mixer Error: RPMDIR not set in configuration

Signed-off-by: Caio Marcelo de Oliveira Filho <caio.oliveira@intel.com>

mixer/cmd/root.go

@@ -31,6 +31,9 @@ var config string
 var RootCmd = &cobra.Command{
 	Use:  "mixer",
 	Long: `Mixer is a tool used to compose OS update content and images.`,
+
+	SilenceUsage:  true,
+	SilenceErrors: true,
 }
 
 type initCmdFlags struct {