Catch Spinx SystemExit when building help

In a recent Sphinx release, it started calling `sys.exit()` and that
caused our whole build process to exit prematurely.

hscommon/sphinxgen.py

@@ -1,9 +1,9 @@
 # Created By: Virgil Dupras
 # Created On: 2011-01-12
 # Copyright 2014 Hardcoded Software (http://www.hardcoded.net)
-# 
-# This software is licensed under the "BSD" License as described in the "LICENSE" file, 
-# which should be included with this package. The terms are also available at 
+#
+# This software is licensed under the "BSD" License as described in the "LICENSE" file,
+# which should be included with this package. The terms are also available at
 # http://www.hardcoded.net/licenses/bsd_license
 
 import os.path as op
@@ -31,7 +31,7 @@ def tixgen(tixurl):
 
 def gen(basepath, destpath, changelogpath, tixurl, confrepl=None, confpath=None, changelogtmpl=None):
     """Generate sphinx docs with all bells and whistles.
-    
+
     basepath: The base sphinx source path.
     destpath: The final path of html files
     changelogpath: The path to the changelog file to insert in changelog.rst.
@@ -66,4 +66,7 @@ def gen(basepath, destpath, changelogpath, tixurl, confrepl=None, confpath=None,
     # missing dependencies which are in the virtualenv). Here, we do exactly what is done when
     # calling the command from bash.
     cmd = load_entry_point('Sphinx', 'console_scripts', 'sphinx-build')
-    cmd(['sphinx-build', basepath, destpath])
+    try:
+        cmd(['sphinx-build', basepath, destpath])
+    except SystemExit:
+        print("Sphinx called sys.exit(), but we're cancelling it because we don't acutally want to exit")