Add structured errors to schema

app/models/error_template.rb

@@ -0,0 +1,3 @@
+class ErrorTemplate < ActiveRecord::Base
+  belongs_to :execution_environment
+end

app/models/error_template_attribute.rb

@@ -0,0 +1,3 @@
+class ErrorTemplateAttribute < ActiveRecord::Base
+  belongs_to :error_template
+end

app/models/structured_error.rb

@@ -0,0 +1,4 @@
+class StructuredError < ActiveRecord::Base
+  belongs_to :error_template
+  belongs_to :file, class_name: 'CodeOcean::File'
+end

app/models/structured_error_attribute.rb

@@ -0,0 +1,4 @@
+class StructuredErrorAttribute < ActiveRecord::Base
+  belongs_to :structured_error
+  belongs_to :error_template_attribute
+end