Make the exception message friendly

[JacksonTian]

the template method is small and smarty, but not friendly for debug. this patch can help developers to debug template. It's useful.

ReferenceError: 4
    2|     <%
    3|     // a comment
 >> 4|     if (data) { data += 12345; }; %>
    5|     <li><%= data %></li>
    6|   

data is not defined
    at eval (eval at <anonymous> (file://localhost/Users/jacksontian/fork/my_underscore/underscore.js:1196:20), <anonymous>:29:9)
    at Function._.template (file://localhost/Users/jacksontian/fork/my_underscore/underscore.js:1202:22)
    at Object.<anonymous> (file://localhost/Users/jacksontian/fork/my_underscore/test/utility.js:193:21)

[jdalton]

I really dig the creativity here. This may not be appropriate for Underscore, but after the v1.0.0-rc.1 bump I'll check into doing something like this with Lo-Dash (which has a build step and can remove it for production builds).

[JacksonTian]

Why not appropriate?

[jashkenas]

Not appropriate because Underscore templates are supposed to be as small and simple as possible. Setting a line number variable on every line of your evaluated template code doesn't fit that mission statement, and may hurt performance of important templates.

Instead, simply precompiling your templates before you send them to the browser is an easier way to get this behavior.

Very cool, though.

[JacksonTian]

Actually, we usually use it at web front-end page, the performance isn't emergency, develop faster will be more important.