Make error handling code cleaner and safer

[mossroy]

The error handling code is not very clean.
We sometimes test on the error message, which is very bad practice
We sometimes put the error message as the article content
We also call alert() from the algorithm, which should not have a dependency on the GUI.

I'm not proud of all this : it's not clean and not bullet-proof

[mossroy]

Almost all of this is now obsolete.
I've created #813 for what's remaining.