valo-cd.xsl lacks error checks

[Tuukka Hastrup]

Currently, valo-cd.xsl does not fail if a resource fails to load. It should be clear when the build is not successful.

When a resource fails to load, xsltproc outputs a warning, and we can terminate using xsl:message such as "Error: Failed to load contents for http://...".

Additionally, if something in the inputs is not as required (empty section, red link in wiki, missing icon for a program etc.), we can append a line to errors.txt.

[Asko Soukka (asko-soukka)]

I'll keep this in mind everytime when refactoring XSLT and will fix this eventually during a longer period.

[Asko Soukka (asko-soukka)]

I guess, we should add a yet another option for build.sh to pass to XSLT to determine whether the XSLT should really be terminated after outputting error message or not.