No error message shown when file not found

[gsingh93]

When a template isn't found, an error should be displayed. Alternatively, constructing a template can return an Option.

[sunng87]

You can now catch render error with a middleware: https://github.com/sunng87/handlebars-iron/blob/master/examples/error.rs

By default, our middleware will return HTTP 500 on render error. Detailed render error will be available when upstream is ready.