Add CI detection for XML syntax errors

[Flyingmana]

While its mainly about detecting errors at all, we should also more specific be able to detect the error fixed in #1901

• detect general xml syntax errors
• detect error from Fix syntax in lib/Zend/Locale/Data/es_419.xml #1901

[fballiano]

@Flyingmana do you have a tool in mind that does xml validation?

[Flyingmana]

not a specific one, but xml validation is a generally solved problem.
I would just start with the first tool an online search returns, and see if it would detect the linked error.

[fballiano]

I'm running:

find . -type f -iname '*.xml' | xargs -I '{}' xmllint --noout '{}'

but it throws errors since a lot of our xml files have this:

./app/code/core/Mage/Core/etc/wsi.xml:2: namespace error : xmlns:typens: 'urn:{{var wsdl.name}}' is not a valid URI
<wsdl:definitions xmlns:typens="urn:{{var wsdl.name}}"

:-\