If no builder is present on an editor, please give a helpful error

[ajepst]

When creating a custom editor, if a builder is not chosen, the error is not very helpful as to the actual problem. The eye-catching part that's up front in the error message (about the argument out of range) feels almost misleadingly unrelated to the missing builder, and the second part that is actually somewhat related doesn't clearly indicate that the builder is completely missing, it sounds like some other sort of policy selection problem.

 System.ArgumentOutOfRangeException
    Specified argument was out of the range of valid values.
    Parameter name: Unable to select a TagBuilderPolicy for subject Profile: Default, Subject: HtmlTags.Conventions.ElementRequest
    at HtmlTags.Conventions.TagCategory.buildPlan(TagSubject subject)

This error message would be more helpful if it instead said something like: "No builder selected for 'MyCustomEditor'. Add something like expr.Always.BuildBy() to fix this error"

Actually putting the 'MyCustomEditor' in the error as shown above would be helpful, but doesn't seem absolutely required if it's difficult.

[jbogard]

I have no idea how we reproduced this now :\