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

hugolib/site.go

@@ -254,9 +254,11 @@ func (s *Site) Build() (err error) {
 	return nil
 }
 
-func (s *Site) Analyze() {
-	s.Process()
-	s.ShowPlan(os.Stdout)
+func (s *Site) Analyze() error {
+	if err := s.Process(); err != nil {
+		return err
+	}
+	return s.ShowPlan(os.Stdout)
 }
 
 func (s *Site) prepTemplates() {