error_uri may be unnecessary for first party apps

[deansaxe]

Section 5.2.2 defines an error_uri. This seems unnecessary - and is defined as optional - since this is only usable by first party apps. This can be removed from the draft since first party app developers should have no need for this data.

[PieterKas]

I believe this is a useful option for developers who may want to provide additional information, or who may want to change that information - this mechanism allows them to more dynamically provide that information.