Validation of metadata to return meaningful messages

## Background
Some validation messages are misleading and rather confuse the user then helping

## Feature
Make the messages more explanatory

## Example
If the default value for an integer field is a binary integer, the message is something in the sense, that 0 cannot be converted to string.

The issue does not name any files, tests, or entry points. Start by locating the metadata validation logic and reproducing the binary-integer default example; done means the validation reports an explanatory metadata error instead of a misleading conversion message, with regression coverage for the case.

Scritto dal modello di indicizzazione a partire dal testo della issue.