drop-ins: check return value

If the function failed, nothing serious would happen
because unlink would probably return EFAULT, but this
would obscure the real error and is a bit sloppy.

Conflicts:
	src/core/unit.c

src/core/unit.c

@@ -2940,6 +2940,9 @@ int unit_remove_drop_in(Unit *u, bool runtime, const char *name) {
         assert(u);
 
         r = drop_in_file(u, runtime, name, &p, &q);
+        if (r < 0)
+                return r;
+
         if (unlink(q) < 0)
                 r = -errno;
         else