[PATCH 6 of 7] templater: specialize ResourceUnavailable error so that it can be caught

[Yuya Nishihara]

# HG changeset patch
# User Yuya Nishihara <yuya at tcha.org>
# Date 1519530630 -32400
#      Sun Feb 25 12:50:30 2018 +0900
# Node ID 02afa73e633146722c187090fd73885fceb552d9
# Parent  35263744d42f5a3c60faec304d044cbee337599d
templater: specialize ResourceUnavailable error so that it can be caught

See the next patch how it will be used.

diff --git a/mercurial/templater.py b/mercurial/templater.py
--- a/mercurial/templater.py
+++ b/mercurial/templater.py
@@ -30,6 +30,9 @@ from . import (
     util,
 )
 
+class ResourceUnavailable(error.Abort):
+    pass
+
 class TemplateNotFound(error.Abort):
     pass
 
@@ -1377,7 +1380,8 @@ class engine(object):
         if v is None:
             v = self._resources.get(key)
         if v is None:
-            raise error.Abort(_('template resource not available: %s') % key)
+            raise ResourceUnavailable(_('template resource not available: %s')
+                                      % key)
         return v
 
     def _load(self, t):