Packaging scripts should terminate on error

The packaging scripts currently don't die on most errors, such as failure to build docs or one or more of Qt's modules. This behaviour makes it likely that packages can be released with pieces missing and testers might not notice, even though the build machines know that there's a problem.

The current behaviour resulted from there being frequent build problems. Perhaps things could be improved by initially implementing an optional strict mode for the packaging scripts. When builds are more reliable, strict mode could be turned on for the packaging machines.