Show notification instead of error when run output shows new version banner

[sidlatau]

Also on another note

..., but I do find the update notification itself helpful. ...

I, too, think that the update notification is not entirely useless. However, If you really wish to keep it, then I reckon we must find a solution to tell the user via vim.notify that there is a new Flutter version instead of letting a command like FlutterRun complete with an error.

Originally posted by @obemu in #470 (comment)