Mms: findPart may still be null

This partially reverts 2757db88a16af8197243025a9393367a6ebba3e1 commit, because
pb.getPartByContentId("<" + src + ">"); may return null. Throw the old IllegalArgumentException
if this happens.

Change-Id: If19b7295131dcc834553b68acc4e25b0fba7dab9
JIRA: CYAN-2100
Issue: https://jira.cyanogenmod.org/browse/CYAN-2100
Signed-off-by: Jorge Ruesga <jorge@ruesga.com>