applets/comic: downgrade severity of plugin log messages

Comic plugins can use a print function to print debug and status output, and some plugins make heavy use of that. Currently, each such message leads to a warning-level log message. These messages are relevant to plugin develpers, but generally not to end users - if there is an error, the comic plugin will report that independently.

Instead, we change the severity for these messages to Info, and set the default severity for printing messages to Warning. This way, they will not be displayed by default, but plugin developers or users interested in these messages for other reasons can still access them independently of the comic applet's internal debug logging.

Along the way, we fix a typo in a log message.