vcs.rb: raise NotFoundError when command not found

* tool/vcs.rb (VCS#get_revisions): raise VCS::NotFoundError when
  command not found.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@61211 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/vcs.rb

@@ -139,6 +139,8 @@ class VCS
           STDERR.reopen NullDevice, 'w'
         end
         self.class.get_revisions(path, @srcdir)
+      rescue Errno::ENOENT => e
+        raise VCS::NotFoundError, e.message
       ensure
         if save_stderr
           STDERR.reopen save_stderr