Change observation logic from other process to threaded loop in same process
Return original response body when response body already exists
Return all error message when unexpected error occured
Suppress unexpected error when pattern doesn't set while validating
Add transaction to create system and stacks at a time
Build stacks in other thread when recreate system
Correspond to add empty array if target not exists in parameters
Unify format for error response
Change to use error message when fail to save in Application PUT API
Merge branch 'feature/optional-pattern' into test/temporary-latest

Conflicts:
	spec/src/models/system_spec.rb
	src/models/system.rb
Merge branch 'feature/fallback' into test/temporary-latest

Conflicts:
	src/controllers/systems_controller.rb