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/check.go

@@ -23,9 +23,13 @@ var check = &cobra.Command{
 	Short: "Check content in the source directory",
 	Long: `Hugo will perform some basic analysis on the content provided
 and will give feedback.`,
-	Run: func(cmd *cobra.Command, args []string) {
-		InitializeConfig()
+	RunE: func(cmd *cobra.Command, args []string) error {
+		if err := InitializeConfig(); err != nil {
+			return err
+		}
 		site := hugolib.Site{}
-		site.Analyze()
+
+		return site.Analyze()
+
 	},
 }