Checks that specified sheets actually exist in the expected workbook 

## Context
While processing the xml tag ```assert-excel``` (codjo-release-test), some sheets were not checked because of leading spaces in their name and no error was thrown.

## Description
The class ```ExcelComparator``` now checks that specified sheets actually exist in the expected workbook. In the error message, missing sheet names are surrounded by simple quotes to show eventually leading/trailing spaces.

Example
Assuming ```WrongSheetName``` is not a sheet in workbook ```expected.xls```, the following xml will fail in release-test :
```xml
<assert-excel expected="expected.xls" sheets="WrongSheetName"/>
```