Speed up Node.FS.EntryProxy.__getattr__() by not spending cycles

Speed up Node.FS.EntryProxy.__getattr__() by not spending cycles
generating an AttributeError exception message that gets ignored most
of the time.  Instead, re-raise an AttributeError subclass that delays
message generation until its __str__() method is actually called.
(Brad Fitzpatrick)

git-svn-id: http://scons.tigris.org/svn/scons/trunk@3785 fdb21ef1-2011-0410-befe-b5e4ea1792b1