Fix arcanist documentation to include "set -e" in example SVN hooks

Summary: "set -e" causes the script to exit if any command returns nonzero.
Without it, we incorrectly discard the failure code. At Facebook everything runs
in "set -e" or some equivalent so I never picked this up in testing.

Test Plan:
Added the hook to my svn local, it blocked bad commits and allowed good ones.

  >>> orbital:~/devtools/svnroot $ svn commit -m "quaa"
  Sending        test.php
  Transmitting file data .svn: Commit failed (details follow):
  svn: Commit blocked by pre-commit hook (exit code 1) with output:

  LINT ERRORS

  This changeset has lint errors. You must fix all lint errors before you can
  commit.

  You can add '@bypass-lint' to your commit message to disable lint checks for
  this commit, or '@nolint' to the file with errors to disable lint for that
  file.

  >>> Lint for test.php:

     Error  (XHP1) PHP Syntax Error!
      This file contains a syntax error: XHPAST Parse Error: syntax error,
      unexpected '}' on line 1

      >>>        1 <?php asdddddd;!}}

Reviewers: svemir, jungejason, tuomaspelkonen, aran

Reviewed By: svemir

CC: aran, svemir, epriestley

Differential Revision: 864

src/docs/userguide/arcanist_hooks.diviner

@@ -3,6 +3,8 @@
 
 Describes how to set up Arcanist as an SVN pre-commit hook.
 
+NOTE: This whole thing might be a little shaky and take a touch of finesse.
+
 = Installing Arcanist SVN Hooks =
 
 You can install Arcanist as an SVN pre-commit hook, to reject commits which
@@ -13,6 +15,7 @@ engines.
 To install Arcanist as a pre-commit hook, add this to your svn/hooks/pre-commit:
 
   #!/bin/sh
+  set -e # Exit with an error code if this fails.
   /usr/local/bin/php -f /path/to/arcanist/bin/arc svn-hook-pre-commit $@ 1>&2
 
 Make sure you make this file executable, or you'll get an error for every commit
@@ -31,6 +34,7 @@ build new linters or customize lint engines. For example, your hook might
 look like this:
 
   #!/bin/sh
+  set -e # Exit with an error code if this fails.
   /usr/local/bin/php -f /path/to/arcanist/bin/arc svn-hook-pre-commit \
     --load-phutil-library=/path/to/custom/lint/engine \
     --load-phutil-library=/path/to/custom/unittest/engine \