Validate the user-specified versions to generate

We allow specifying which versions we want to generate, but without validation,
we would end up with an empty list, which is not what the user meant if they
mistyped a version.

Author: carlosmn

lib/docurium.rb

@@ -125,7 +125,16 @@ def generate_docs(options)
     @tf = File.expand_path(File.join(File.dirname(__FILE__), 'docurium', 'layout.mustache'))
     versions = get_versions
     versions << 'HEAD'
-    versions = versions & options[:for] unless options[:for].empty?
+    # If the user specified versions, validate them and overwrite
+    if !(vers = options[:for]).empty?
+      vers.each do |v|
+        next if versions.include?(v)
+        puts "Unknown version #{v}"
+        exit(false)
+      end
+      versions = vers
+    end
+
     nversions = versions.size
     output = Queue.new
     pipes = {}