fstab-generator: don't skip /usr handling if root handling didn't work correctly

Let's follow the same logic for all mounts here: log errors, and exit the
process uncleanly ultimately, but do not skip further mounts if we encounter a
problem with an earlier one.

Fixes: #2344

src/fstab-generator/fstab-generator.c

@@ -683,10 +683,15 @@ int main(int argc, char *argv[]) {
 
         /* Always honour root= and usr= in the kernel command line if we are in an initrd */
         if (in_initrd()) {
+                int k;
+
                 r = add_sysroot_mount();
-                if (r == 0)
-                        r = add_sysroot_usr_mount();
-        }
+
+                k = add_sysroot_usr_mount();
+                if (k < 0)
+                        r = k;
+        } else
+                r = 0;
 
         /* Honour /etc/fstab only when that's enabled */
         if (arg_fstab_enabled) {