Fail rather than emitting a deprecation warning when the value of a `Property` typed task property is changed after the task starts execution. Also include the property display name, if attached, in the 'cannot change the value' error messages.
Generate a basic display name for `Property` instances created as the value for a read-only managed property. Use the display name in 'this property has no value' error messages to help with problem diagnosis. The display name could include some more context, could be used in more error messages, and be applied to more things. This commit is simply an incremental improvement and later changes will improve diagnostics further.