Parser syntax errors should be passed to the callback, not thrown

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Originally submitted as akdubya/dustjs#12:

> When the parser encounters a syntax error, it currently throws a SyntaxError exception instead of bubbling it up to the render callback like I'd expect. Catching this exception is proving to be a challenge in an Express app. I'd much rather be able to handle it in my callback.
>
> This will throw a SyntaxError in Node (run it as a file, though; the REPL catches and hides the exception):
>
> ``` js
> var dust = require('dust'),
> context = dust.makeBase();
>
> dust.renderSource('{ if (err) {
> console.log('I got an error!'); // never gets here
> }
> });
> ```

Start by running the supplied Node example and tracing the dust.renderSource callback and parser error path. Confirm the syntax error currently escapes as a throw, then verify that the render callback receives the error instead and that the example logs “I got an error!”.

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