New feature to add warnings to computed fields

# New feature to add warnings to computed fields
## Context
For some computed fields, we needed to add warnings and continue the export normally.

## Description
Developers implementing a computed field can now use one of the `AbstractComputedField.appendWarning` methods in order  to add warnings to a computed field.
A reserved computed field has been added to store these warnings.

## Example
The following code :
```java
public class NavDifferencePercentComputedField extends AbstractComputedField {
  public void compute(ComputedContext computedContext, Connection connection) 
                    throws SQLException {
    String query = "MERGE INTO " + computedContext.getComputedTableName() + " comp "
           + "USING ( select  PET_NAME, PET_BIRTH_DATE "
           + "from " + computedContext.getSelectionTableName() ) selectdata "
           + "ON (comp.SELECTION_ID = selectdata.SELECTION_ID) "
           + "WHEN MATCHED THEN "
           + "UPDATE SET PET_AGE = "
			+ "   case when NB_YEARS(PET_BIRTH_DATE, NOW) < 15 "
			+ "   then NB_YEARS(PET_BIRTH_DATE, NOW) "
			+ "   else null"
			+ "   end "
            + appendWarning("NB_YEARS(PET_BIRTH_DATE, NOW) < 15", "It's very old", 
                            "Warnings for pet named ' || PET_NAME || ' :\n");

    String sql = computedContext.replaceVariables(query);
    PreparedStatement statement = connection.prepareStatement(sql);
    try {
      statement.executeUpdate();
    } finally {
      statement.close();
    }
  }
```
could generate a warning message like the following one :
```
Warnings for pet named Felix :
It's very old
```
Such message is visible in the workflow logs screen.

# Improvement of exception messages thrown by `SqlUtil.loadquery`
## Context
When a resource is not found by `SqlUtil.loadquery` methods, the thrown exception wasn't explicit.

## Description
On failure, `SqlUtil.loadquery` methods throw an exception with a more meaningful message.