Fix Status::getMessage accidentially returning string instead of Message

Fix Status::getMessage accidentially returning string instead of Message

This mistake was introduced in commit
92e284d3fa62f45e20fed34c4359c575481d583c and the reason for the two
disabled tests. I did not enabled the second test because of an
unrelated problem. The first enabled test already covers the fix.

The method should return Message objects only but did return a
string in that special case (multiple warnings set but no context
message key).

Unfortunatelly Status::getMessage does have many, many more
problems but I understand it's not a good idea to address them all
in a single confusing patch.

Change-Id: I0dc37e248f407019d5921aaaca3eabba338b0fd3