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/version.go

@@ -32,7 +32,7 @@ var version = &cobra.Command{
 	Use:   "version",
 	Short: "Print the version number of Hugo",
 	Long:  `All software has versions. This is Hugo's.`,
-	Run: func(cmd *cobra.Command, args []string) {
+	RunE: func(cmd *cobra.Command, args []string) error {
 		if hugolib.BuildDate == "" {
 			setBuildDate() // set the build date from executable's mdate
 		} else {
@@ -43,6 +43,8 @@ var version = &cobra.Command{
 		} else {
 			fmt.Printf("Hugo Static Site Generator v%s-%s BuildDate: %s\n", helpers.HugoVersion(), strings.ToUpper(hugolib.CommitHash), hugolib.BuildDate)
 		}
+
+		return nil
 	},
 }