dont use datatemplate in datavalidationerrors.

[danwalmsley]

What does the pull request do?

remove use of datatemplates in DataValidationErrors.

What is the current behavior?

users cannot provide their own template easily.

What is the updated/expected behavior with this PR?

User can supply a template simply like:

<Application.DataTemplates>
    <local:ViewLocator/>
    <DataTemplate DataType="models:ErrorDescriptor" x:DataType="models:ErrorDescriptor">
      <TextBlock Text="Error here"/>
    </DataTemplate>

How was the solution implemented (if it's not obvious)?

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/Avaloniaui.net with user documentation

Breaking changes

Obsoletions / Deprecations

Fixed issues