macdeployqt does not check result of running hdiutil

macdeployqt runs hdiutil to create dmg image, but does not check the result. When hdiutil fails, macdeployqt stays quiet and acts as if nothing happened.

One case when hdiutil fails seems to be when the archive is large enough (85M seems "largs"). I tested this in the Travis-CI mac environment. I had to run hdiutil manually and pass -size 130m to get the image.

This is tested on 5.2.1 available on homebrew, but seems to be the case even for 5.6.0 (I checked the code and didn't find a check for the error either).

Here is a log with hditool verbose output, as is run by macdeployqt
https://travis-ci.org/openstreetmap/merkaartor/jobs/134161821

Here is a log with -size 130m added:
https://travis-ci.org/openstreetmap/merkaartor/jobs/134173120