Fix some problems with project loading

* build/targets.jam
  (project-target.find): Better error reporting.

* build/project.jam
  (load-jamfile): Use 'path.native' when loading Jamfile,
   otherwise, target id with absolute directory name won't
   work for Windows.


[SVN r26271]

tools/build

@@ -1 +1 @@
-Subproject commit 6b0bc477f94a38fb0425484bf4fa0baf71ddc468
+Subproject commit 094b2aa6566fa8d4dda8f2ea0992e385ec0a246a