extmgr: add nested exception message into new exception

this enables to see helpful errors while not in debug mode.

Topic: AAA
Bug-Url: https://bugzilla.redhat.com/show_bug.cgi?id=1120720
Change-Id: If321921682d4de599db202956babb1af8a7b315b
Signed-off-by: Alon Bar-Lev <alonbl@redhat.com>