Markdown: Redirect to show source file if parsing fails

If the file cannot be parsed as markdown redirect the browser
to view the source of the file. This will at least permit the
end user to read the raw contents, instead of getting a 500
error with no links forward.

Change-Id: I124349dbe1fedb9203699653b802f19ad58f4baa
diff --git a/gitiles-servlet/src/main/java/com/google/gitiles/doc/DocServlet.java b/gitiles-servlet/src/main/java/com/google/gitiles/doc/DocServlet.java
index 188d503..d4ae488 100644
--- a/gitiles-servlet/src/main/java/com/google/gitiles/doc/DocServlet.java
+++ b/gitiles-servlet/src/main/java/com/google/gitiles/doc/DocServlet.java
@@ -115,7 +115,7 @@
           view, srcmd.path,
           srcmd.read(rw.getObjectReader(), inputLimit));
       if (doc == null) {
-        res.setStatus(SC_INTERNAL_SERVER_ERROR);
+        res.sendRedirect(GitilesView.show().copyFrom(view).toUrl());
         return;
       }