New feature to add warnings to computed fields

new feature to add warnings for computed fields 

## Context
When the value of a computed field can't be computed, we needed to add warnings while still continue normally the export.
When a resource is not found by `SqlUtil.loadquery` methods, the thrown exception wasn't clear.

## Description
Developers implementing a computed field can now uses one the `AbstractComputedField.appendWarning` methods to add warnings for a computed field.
On failure, the `SqlUtil.loadquery` methods throws an exception with a more meaningful message.