[gfortran,patch] Improve error messages for bounds-checking and a few other cases

[FX Coudert]

> I have just put your patch on to regtest, whilst I do some  
> gardening.  Are you going to submit some testscases?  I'll see if I  
> can figure out the two mystery cases and will be back to you  
> tonight or tomorrow morning.

I'm trying to trigger those while watching Roland-Garros (which is a  
shame, because I could actually *be* there), but so far, I've had no  
luck.

FX