Message when only 1 child is expected

[NiedziolkaMichal]

When schema expects a single child from a range of options, validated element with no children throws error saying that "one or more" children are acceptable.

For example this schema expects test element to have either inner1 or inner2 child:

element test { (element inner1 { text } | element inner2 { text }) }

When element test is created with no child inside, following message is used:

Element test is missing a required instance of one or more of the following child elements: inner1, inner2.

More appropriate message would be:

Element test is missing a required instance of one of the following child elements: inner1, inner2.