Extend the error when the project is not found at include:file

ee/spec/services/ci/create_pipeline_service/compliance_spec.rb

@@ -63,7 +63,7 @@ RSpec.describe Ci::CreatePipelineService do
 
   context 'when user does not have access to compliance project' do
     it 'includes access denied error' do
-      expect(execute.payload.yaml_errors).to eq "Project `compliance/hippa` not found or access denied!"
+      expect(execute.payload.yaml_errors).to eq "Project `compliance/hippa` not found or access denied! Make sure any includes in the pipeline configuration are correctly defined."
     end
 
     it 'does not persist jobs' do

lib/gitlab/ci/config/external/file/project.rb

@@ -31,7 +31,7 @@ module Gitlab
 
             def validate_content!
               if !can_access_local_content?
-                errors.push("Project `#{project_name}` not found or access denied!")
+                errors.push("Project `#{project_name}` not found or access denied! Make sure any includes in the pipeline configuration are correctly defined.")
               elsif sha.nil?
                 errors.push("Project `#{project_name}` reference `#{ref_name}` does not exist!")
               elsif content.nil?