Improve the status and error handling/notification

[jonasraoni]

Looks like users have no clue about what's happening in the plugin, if there's something missing to make it work, last time it was executed, if it's being executed perfectly (and if not, they can't see the errors), etc.

Part of this task should be probably synchronized with the pkppln repository, which could probably forward some helpful (and not compromising) error messages/hints, e.g. "failed to collect the deposit due to a request failure accessing the URL xxx".

We can also try to add some helpful instructions.

The main objective is to decrease the noise with error reports from users.

[jonasraoni]

Partially done with #20 (better error messages).