templater: add exception-raising version of open_template()

I'm about to add another caller of `open_template()` (in the template
loader). That caller will want to get exceptions instead of `(None,
None)` if the template doesn't exist. This patch therefore changes
`open_template()` to raise exceptions and adds a new
`try_open_template()` that returns the `(None, None)` value.

Differential Revision: https://phab.mercurial-scm.org/D8905

mercurial/debugcommands.py

@@ -1672,7 +1672,7 b' def debuginstall(ui, **opts):'
     fm.write(b'templatedirs', b'checking templates (%s)...\n', p or b'')
     fm.condwrite(not p, b'', _(b" no template directories found\n"))
     if p:
-        (m, fp) = templater.open_template(b"map-cmdline.default")
+        (m, fp) = templater.try_open_template(b"map-cmdline.default")
         if m:
             # template found, check if it is working
             err = None

mercurial/formatter.py

@@ -600,9 +600,9 b' def lookuptemplate(ui, topic, tmpl):'
 
     # perhaps a stock style?
     if not os.path.split(tmpl)[0]:
-        (mapname, fp) = templater.open_template(
+        (mapname, fp) = templater.try_open_template(
             b'map-cmdline.' + tmpl
-        ) or templater.open_template(tmpl)
+        ) or templater.try_open_template(tmpl)
         if mapname:
             return mapfile_templatespec(topic, mapname, fp)
 

mercurial/hgweb/hgweb_mod.py

@@ -78,7 +78,7 b' def _stylemap(styles, path=None):'
         locations = (os.path.join(style, b'map'), b'map-' + style, b'map')
 
         for location in locations:
-            mapfile, fp = templater.open_template(location, path)
+            mapfile, fp = templater.try_open_template(location, path)
             if mapfile:
                 return style, mapfile, fp
 

mercurial/logcmdutil.py

@@ -628,9 +628,9 b' def _lookuptemplate(ui, tmpl, style):'
         mapfile = style
         fp = None
         if not os.path.split(mapfile)[0]:
-            (mapname, fp) = templater.open_template(
+            (mapname, fp) = templater.try_open_template(
                 b'map-cmdline.' + mapfile
-            ) or templater.open_template(mapfile)
+            ) or templater.try_open_template(mapfile)
             if mapname:
                 mapfile = mapname
         return formatter.mapfile_templatespec(b'changeset', mapfile, fp)

mercurial/templater.py

@@ -1095,17 +1095,18 b' def open_template(name, templatepath=Non'
         templatepath = templatedir()
     if templatepath is not None or os.path.isabs(name):
         f = os.path.join(templatepath, name)
-        try:
             return f, open(f, mode='rb')
-        except EnvironmentError:
-            return None, None
     else:
         name_parts = pycompat.sysstr(name).split('/')
         package_name = '.'.join(['mercurial', 'templates'] + name_parts[:-1])
-        try:
             return (
                 name,
                 resourceutil.open_resource(package_name, name_parts[-1]),
             )
-        except (ImportError, OSError):
+
+
+def try_open_template(name, templatepath=None):
+    try:
+        return open_template(name, templatepath)
+    except (EnvironmentError, ImportError):
             return None, None