restrict rescue-clause in IncludeTag#render

As it previously enclosed the whole method, it also
wrapped file validation errors, which is not meant to be.

Fixes #1745.

lib/jekyll/tags/include.rb

@@ -96,6 +96,7 @@ eos
         path = File.join(dir, file)
         validate_file(path, context.registers[:site].safe)
 
+        begin
           partial = Liquid::Template.parse(source(path, context))
 
           context.stack do
@@ -105,6 +106,7 @@ eos
         rescue => e
           raise IncludeTagError.new e.message, File.join(INCLUDES_DIR, @file)
         end
+      end
 
       def validate_dir(dir, safe)
         if File.symlink?(dir) && safe