Simplify expression refactoring is failing due to extended operands.

Fixed several problems:
- Fixed the failure cause in SimplifyExpressionRefactoring
- After one visitor failed, AutoRefactor stopped applying any changes
- Now report errors to the user
- Now log AND report error to the user

SimplifyExpressionSample.java:
Added code triggerring the failure.

SimplifyExpressionRefactoring.java:
In getNullCheckedExpression(), fixed the bug.
In checkNoExtendedOperands(), improved error reporting.
Code cleanup + formatting.

ASTHelper.java:
Added getSourceLocation().

PreferenceHelper.java:
In getBoolean(), fixed a bug that prevented useful use of the debug mode.

AggregateASTVisitor.java:
Removed a useless constructor.

RefactoringsTest.java:
In log*(), first log the error, then throw it.
Consequence of the change to AggregateASTVisitor.

AutoRefactorHandler.java, ChooseRefactoringsWizardHandler.java:
Called UIHelper.showErrorDialog().

UIHelper.java: ADDED