add a dry-run option to handlers

[mfrasca]

some handlers (specifically the sentryHandler) need check or load packages in order to run. or they might want to test for the presence of all needed parameters. it would be nice to have a dry-run option that informs the handler action not to do anything except check the environment.

So you would be able to follow the workflow:

• create the handler
• ask it whether it can function
• if yes, continue attaching it to a logger
• if no, take alternative measures

without this dry-run option, you are here:

• create the handler
• attach it to a logger
• when the handler is activated, fall in some error situation if things are not in order