Specified response codes for error handler
Functional snapshot file controller
Services throw NotFoundException instead of returning null
Services for snapshot file controller
Merge branch 'refactor' of github.com:kesapojat/tmc-snapshot-api into refactor

Conflicts:
	src/main/java/fi/helsinki/cs/tmc/snapshot/api/service/CourseServiceImpl.java
	src/main/java/fi/helsinki/cs/tmc/snapshot/api/service/ExerciseServiceImpl.java
	src/main/java/fi/helsinki/cs/tmc/snapshot/api/util/EventTransformer.java
Merge branch 'refactor' of github.com:kesapojat/tmc-snapshot-api into refactor