Project root loading tweaks.

    * new/project-root.jam (load): Really produce an error message when
        project root file is not found.
    * test/project_test3.py: Test for the error reporting.


[SVN r14280]

tools/build

@@ -1 +1 @@
-Subproject commit ea18519ec7dfc016665e7dcfd60ebf4a44681346
+Subproject commit c9c1a714d431f3ab8aba1b8304e4079d612e76e9