rename errorMessage with an assertJ name

Hi,

I have a bean : 
```java
public class SessionBean {

    /**
     * sessionKey opened on geohub.
     */
    private String sessionKey;

    /**
     * url of my awesome service.
     */
    private String url;

    /**
     * Error message if any.
     */
    private String errorMessage;
}
```

and the generated file contains:

```java
    public SessionBeanAssert hasErrorMessage(String errorMessage)
    {
        // check that actual SessionBean we want to make assertions on is not null.
        isNotNull();

        // we overrides the default error message with a more explicit one
        String errorMessage = "\nExpected errorMessage of:\n  <%s>\nto be:\n  <%s>\n but was:\n  <%s>";

        // check
        if (!actual.getErrorMessage().equals(errorMessage)) {
            failWithMessage(errorMessage, actual, errorMessage, actual.getErrorMessage());
        }

        // return the current assertion for method chaining
        return this;
    }
```

with a compilation error.

I propose to rename the internal field with an assertj name.