trivial: Set the source file before parsing

This is so it can exist in error messages.
trivial: Add AS_NODE_ERROR_INVALID_MARKUP for a less fatal error
Markup errors should not be fatal when assembling a store

If there's a single AppData file with invalid markup, do not fail to add all
applications to the store and instead just show a warning.

This fixes some odd behaviour in gnome-software on Fedora 21 when old versions
of the transmageddon package are also installed.