SLING-3102 - Inconsistent behaviour when loading archetypes in new Sling
bundle project wizard

Rework error handling in the project creation wizards and pages.

git-svn-id: https://svn.apache.org/repos/asf/sling/branches/tooling-ide-vlt@1525867 13f79535-47bb-0310-9956-ffa450edef68