More clear error messages in http responses

[sharonovd]

Currently, /up http handler provides unclear error messages when something goes sour (basically, 500 with "something went wrong text"). It would be nice to have detailed error report (which migration failed on which instance, and which actually applied) available both in logs AND http response body

[denesterov]

Besides that, an arbitrary text message as a result of (succeeded) migration is also would be be great. Example use case: migration estimating change size or reporting about job results.