Improve error handling in commands

Cobra, the CLI commander in use in Hugo, has some long awaited improvements in the error handling department.
This enables a more centralized error handling approach.

This commit introduces that by changing all the command funcs to `RunE`:

* The core part of the error logging, usage logging and `os.Exit(-1)` is now performed in one place and that one place only.
* The usage text is now only shown on invalid arguments etc. (user errors)

Fixes #1502

commands/import.go

@@ -1,31 +0,0 @@
-// Copyright © 2015 Steve Francia <spf@spf13.com>.
-//
-// Licensed under the Apache License, Version 2.0 (the "License");
-// you may not use this file except in compliance with the License.
-// You may obtain a copy of the License at
-// http://www.apache.org/licenses/LICENSE-2.0
-//
-// Unless required by applicable law or agreed to in writing, software
-// distributed under the License is distributed on an "AS IS" BASIS,
-// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
-// See the License for the specific language governing permissions and
-// limitations under the License.
-
-package commands
-
-import (
-	"github.com/spf13/cobra"
-)
-
-var importCmd = &cobra.Command{
-	Use:   "import",
-	Short: "Import your site from others.",
-	Long: `Import your site from other web site generators like Jekyll.
-
-Import requires a subcommand, e.g. ` + "`hugo import jekyll jekyll_root_path target_path`.",
-	Run: nil,
-}
-
-func init() {
-	importCmd.AddCommand(importJekyllCmd)
-}