I wrote a Custom Authentication Provider Plug-in for OAuth authentication with a SAAS service. It works fine. 

Question: How do I give error messages to the user if there is a problem during the OAuth authentication and authorization dance? Eg if the client id or secret is wrong. Or if the user declined to authorize the app when asked? 

Thanks.

PS. pages that were helpful:
http://help.salesforce.com/apex/HTViewHelpDoc?id=sso_provider_plugin_custom.htm&language=en_US#plugin_create
https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/authproviderplugin.htm

I wrote a Custom Authentication Provider Plug-in for OAuth authentication with a SAAS service. It works fine. 

Question: How do I give error messages to the user if there is a problem during the OAuth authentication and authorization dance? Eg if the client id or secret is wrong. Or if the user declined to authorize the app when asked? 

Thanks.

PS. pages that were helpful:
http://help.salesforce.com/apex/HTViewHelpDoc?id=sso_provider_plugin_custom.htm&language=en_US#plugin_create
https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/authproviderplugin.htm