installer: improved error handling

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@2856 1f8fe7d8-47f0-464c-8d0a-336f4953ab05
optimized asset image thumbnail creation (+fixed bug in combination with GD Adapter)

git-svn-id: http://www.pimcore.org/svn/pimcore/private/core/trunk@2857 1f8fe7d8-47f0-464c-8d0a-336f4953ab05