appstream-util validate should fail for unsupported HTML tags

[sonnyp]

Only <p/>, <ul/>, <ol/>, <em/> and <code/> are supported in the description tag.

When other tags are present, appstream-util validate should fail.

[ximion]

appstream-util is a tool from a different project, appstream-glib, not from this project.

If I validate with appstreamcli, invalid markup is complained about:

$ appstreamcli validate org.example.TestApp.metainfo.xml
E: org.example.TestApp:101: description-para-markup-invalid pre

Validation failed: errors: 1

So, please report this issue against appstream-glib: https://github.com/hughsie/appstream-glib/issues

[sonnyp]

hughsie/appstream-glib#322