add source filename info whe evaluating .jbuilder files

It's quite hard to find errors in .jbuilder files. When there is an error in it, the stacktrace looks like:

```
NoMethodError - undefined method `id' for nil:NilClass:
  tilt-jbuilder (0.6.0) lib/tilt/jbuilder.rb:109:in `block (4 levels) in evaluate'
  ...
```

Where line 109 doesn't even exist in jbuilder.rb file. No clue where the error can be... :-(

With my modification the stacktrace looks like:

```
NoMethodError - undefined method `id' for nil:NilClass:
  app/views/api/product.jbuilder:63:in `block (4 levels) in evaluate'
  ...
```

we now know exactly where the error is!