New gracefulExit MOJO option to exit the Juliac MOJO without throwing an...

New gracefulExit MOJO option to exit the Juliac MOJO without throwing an exception even if errors are reported.

This feature originates from a discussion with Nicolas Salatge and Julien Lesbegueries from Petals Link, who want to be able to run commands like <code>mvn eclipse:eclipse</code> even if this plugin reports an error. The issue is not specific to this plugin but to the fact that, in most cases, this plugin is associated with the <code>generate-sources</code> lifecycle phase which is run when commands like <code>mvn eclipse:eclipse</code> are invoked.