templater: selecting a style with no templates does not crash (issue4140)

Running `hg log --style compact` (or any other style) raised a traceback when
no template directory was there. Now there is a message:

Abort: style 'compact' not found
(available styles: no templates found, try `hg debuginstall` for more info)


There is no test because this would require to rename the template directory.
But this would influence other tests running in parallel. And when the test
would be aborted the wrong named directory would remain, especially a problem
when running with -l.

mercurial/templater.py

@@ -447,8 +447,10 b' class engine(object):'
 engines = {'default': engine}
 
 def stylelist():
-    path = templatepath()[0]
-    dirlist =  os.listdir(path)
+    paths = templatepath()
+    if not paths:
+        return _('no templates found, try `hg debuginstall` for more info')
+    dirlist =  os.listdir(paths[0])
     stylelist = []
     for file in dirlist:
         split = file.split(".")