Better error message for FileAllocator failure

In database_holder.cpp we have the following code:

        if (txn->lockState()->isWriteLocked() && FileAllocator::get()->hasFailed()) {

            uassert(17507, "Can't take a write lock while out of disk space", false);

        }

If FileAllocator may fail for other reasons (e.g.: the dbpath is on a network drive and there was a network problem) then this message is misleading.

Something like "Can't take a write lock: FileAllocator failed" would be a better message; if we could provide more info about the failure somehow, that would be even better.