The E4 model contains a "Position in list" element which uses a specific
syntax. If the syntax is violated a confusing message is logged using
System.err: "Invalid list position."

This patch:

* Improves the information in the message
* Logs to the log instead of System.err

Change-Id: I5a498cc2ea8706bfdf9bafc1f5f02c1864e0fdfd
Signed-off-by: Wim Jongman <wim.jongman@remainsoftware.com>
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.ui/+/176567
Tested-by: Platform Bot <platform-bot@eclipse.org>
Reviewed-by: Alexander Kurtakov <akurtako@redhat.com>