Exception should be replaced by error message when database is down

42 r16833

I've tried a reboot of the db server while a jahia is running. There was no problem : the jahia server is down as long as db is unreachable (not a surprise) and returns to life as soon as db is back.
There's just a litle problem: while db is down, each request on jahia server generates an exception, what spams the console. Should be replaced by an error message that informs that db is down. If possible, the best solution would be that we introduce a boolean wich store the fact we have already detected that db is down, so that the first request displays the message in INFO mode, and all the followings display it only in DEBUG mode. And first successful request invert boolean value.