Improve error logs to help locating the issue in the HTL template

When there is something wrong in the htl template (like trying to access it.dummy.property which does not exist for exampl), the error log you get in the whole Petridish setup (and probably the same in the production setup) is something like:

```
[hlx] error: Error while rendering the resource: TypeError: Cannot convert undefined or null to object
at Function.keys ()
at lengthOf (../project-helix/modules/helix-helpx/.hlx/build/html.js:295:53)
at ../project-helix/modules/helix-helpx/.hlx/build/html.js:442:28
at Generator.next ()
at onFulfilled (../project-helix/modules/helix-helpx/node_modules/co/index.js:65:19)
at ../project-helix/modules/helix-helpx/node_modules/co/index.js:54:5
at new Promise ()
at co (../project-helix/modules/helix-helpx/node_modules/co/index.js:50:10)
at Runtime.run (../project-helix/modules/helix-helpx/node_modules/@adobe/htlengine/src/runtime/Runtime.js:47:12)
at run (../project-helix/modules/helix-helpx/.hlx/build/html.js:305:20)
at main (../project-helix/modules/helix-helpx/.hlx/build/html.js:482:12)
at getContextPath.catch.then (../project-helix/modules/helix-helpx/.hlx/build/html.js:240:19)
at
at process._tickCallback (internal/process/next_tick.js:182:7)
```

Which gives not hint on what is wrong in the template. It is crucial for development to output a meaningful error that helps locating the issue.