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/genautocomplete.go

@@ -31,16 +31,19 @@ or just source them in directly:
 
 	$ . /etc/bash_completion`,
 
-	Run: func(cmd *cobra.Command, args []string) {
+	RunE: func(cmd *cobra.Command, args []string) error {
 		if autocompleteType != "bash" {
-			jww.FATAL.Fatalln("Only Bash is supported for now")
+			return newUserError("Only Bash is supported for now")
 		}
+
 		err := cmd.Root().GenBashCompletionFile(autocompleteTarget)
+
 		if err != nil {
-			jww.FATAL.Fatalln("Failed to generate shell completion file:", err)
+			return err
 		} else {
 			jww.FEEDBACK.Println("Bash completion file for Hugo saved to", autocompleteTarget)
 		}
+		return nil
 	},
 }