tools: Distingush empty or failed runs of godoc.

- godoc doesn't always have good results.

eris/tools.py

@@ -476,7 +476,9 @@ def godoc(path):
         os.symlink(os.path.abspath(path), symlink_path)
         stdout, stderr, returncode = _do_command(["go", "doc", "."], cwd=temp_dir)
         os.remove(symlink_path)
-    return Status.normal, stdout
+    status = (Status.not_applicable if stdout.strip() == "" or returncode != 0
+              else Status.normal)
+    return status, stdout + stderr
 
 
 @deps(deps={"git"}, url="https://git-scm.com/docs/git-log",