Multiple error notifications

[aiscarvalho]

There should only be one error notification visible at each time.

At the moment, it's possible to get an infinite number of notification of the same error: if the user clicks the button(s) that trigger the error, each click creates a new error notification. Since the error notification is intended for the user, and displays an informative but generic message, there's no need for repetition.

[basilesimon]

Not sure I agree with the bug label here – it's working very much as expected.

Which isn't to say that the output is wholly desirable. Maybe clicking dangerous side effect buttons (e.g. "update contribution") should disable the button(s) for a set amount of time?