Make Metadata Display preview expose errors to user and also render output natively

[DiegoPino]

What?

The new RC2 Metadata Display preview is great. Now we need to make it better. What I will add is:

1. An end format aware preview. If HTML is selected the output should be rendered HTML
2. Parsers/validators. XML selected? JSON-LD? Make sure the output is compliant, and if not, tell the user? HTML? We can even use https://github.com/validator/validator/wiki/Service-»-Input-»-POST-body
3. Errors? Right now it simply fails (not so nice). Let's expose the errors to the user so those can be corrected.

[DiegoPino]

Solved via 96faf58