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

[François-Xavier Coudert]

> Commited on mainline. I don't think it's necessary to backport to
> 4.1, I know (from off-list mail) that Paul thinks otherwise.

Unless someone strongly objects, I will backport this to the 4.1
branch. Bounds-checking is mostly broken in its current state and I
don't feel like devising two different series of fixes for both
branch.

FX