[args] Ensure gn tracelog is processed when non-fatal error happens.

Before this change, gn will not save tracelog when a warning occurs.
This change fixes this issue.

Change-Id: Ib3a690a200aa228532ee4ff5b77fa63ebfaa3042
Bug: crbug.com/gn/98
Reviewed-on: https://gn-review.googlesource.com/c/gn/+/5700
Reviewed-by: Brett Wilson <brettw@chromium.org>
Commit-Queue: Brett Wilson <brettw@chromium.org>

tools/gn/setup.cc

diff --git a/tools/gn/setup.cc b/tools/gn/setup.cc
index f3db362..ef26431 100644
--- a/tools/gn/setup.cc
+++ b/tools/gn/setup.cc
@@ -400,7 +400,7 @@
     OutputString(
         "\nThe build continued as if that argument was "
         "unspecified.\n\n");
-    return true;
+    // Nonfatal error.
   }
 
   if (check_public_headers_) {