A follow-up to #179, this PR changes the gridsync.msg module to no longer automatically log every message that's shown to users, and adds distinct/explicit logging.info, logging.critical, logging.error calls in places where logging is actually desired. Distinguishing between these two types of messages -- i.e., QMessageBox-based messages and logging-based messages -- allows potentially sensitive information (such as folder names, paths, etc.) to be shown in the former, while omitting it from the latter. Normal, user-facing "error" messages that are shown as a result of a user trying to add a file instead of a folder or a folder which already exists, for example, will thus no longer result in the names of those files/folders also showing up in the logs.