Box Handlebars template error

See https://github.com/azerupi/mdBook/pull/466#issuecomment-335450110

src/lib.rs

@@ -105,7 +105,7 @@ pub mod errors {
         foreign_links {
             Io(::std::io::Error);
             HandlebarsRender(::handlebars::RenderError);
-            HandlebarsTemplate(::handlebars::TemplateError);
+            HandlebarsTemplate(Box<::handlebars::TemplateError>);
             Utf8(::std::string::FromUtf8Error);
         }
 
@@ -116,4 +116,11 @@ pub mod errors {
             }
         }
     }
+
+    // Box to halve the size of Error
+    impl From<::handlebars::TemplateError> for Error {
+        fn from(e: ::handlebars::TemplateError) -> Error {
+            From::from(Box::new(e))
+        }
+    }
 }