Give some helpful error-text when `arc diff` fails with a 413.
Give some helpful error-text when `arc diff` fails with a 413.

Summary:
I tried to do this via a python wrapper in D51470, but redirecting
stdout/stderr there proved too hard.  Let's just hack the actual phab
source instead.

Test Plan:
In the react-render-server repo I did
   git co b6cfc39
   react-render-server% arc diff --verbatim --nolint --nounit --excuse testing HEAD^
and saw it say:
```
 Exception
[HTTP/413]
<html>
<head><title>413 Request Entity Too Large</title></head>
<body bgcolor="white">
<center><h1>413 Request Entity Too Large</h1></center>
<hr><center>nginx</center>
</body>
</html>
(Run with `--trace` for a full exception trace.)
(Try `arc diff --less-context` to work around this error.)
```
That last line is my new content!

Reviewers: benkraft

Reviewed By: benkraft

Differential Revision: https://phabricator.khanacademy.org/D51973

Khan/arcanist,Khan/arcanist,Khan/arcanist,Khan/arcanist