Ci::YamlProcessor#validation_errors method for structured output

- Added a new method to return a list of errors instead of a single
  error string.
- Added simple join of structured error messages in Ci::LintsController
- Repeats some processing, but to avoid this would require refactoring
  exception raising and rescuing out of the entire class and all of its
  invocations across the application.
- Existing echnical debt issue addresses the overall problem:
  https://gitlab.com/gitlab-org/gitlab/issues/30066

app/controllers/projects/ci/lints_controller.rb

@@ -8,7 +8,7 @@ class Projects::Ci::LintsController < Projects::ApplicationController
 
   def create
     @content = params[:content]
-    @error = Gitlab::Ci::YamlProcessor.validation_message(@content, yaml_processor_options)
+    @error  = Gitlab::Ci::YamlProcessor.validation_errors(@content, yaml_processor_options).join(', ')
     @status = @error.blank?
 
     if @error.blank?

lib/gitlab/ci/yaml_processor.rb

@@ -102,6 +102,20 @@ module Gitlab
         end
       end
 
+      def self.validation_errors(content, opts = {})
+        return ['Please provide content of .gitlab-ci.yml'] if content.blank?
+
+        config = Gitlab::Ci::Config.new(content, **opts)
+        return config.errors unless config.valid?
+
+        begin
+          Gitlab::Ci::YamlProcessor.new(content, opts)
+          []
+        rescue ValidationError => e
+          [e.message]
+        end
+      end
+
       private
 
       def initial_parsing