Consider adding callback for load or reload errors for all providers

[ycrumeyrolle]

Extension of the issue #489.
As for now, it is possible to handle exception occurring on load only for file-based providers.

            builder.AddJsonFile("settings.json")
                   .SetFileLoadExceptionHandler(ctx =>
                   {
                       // handle error
                   });

Exceptions may occurs with every provider, especially with remote providers like Azure Key Vault & Redis providers. The SetFileLoadExceptionHandler() extension method should be replaced by a more generic SetLoadExceptionHandler() method.

Question : Can the handler be generic enough for been common to all providers ?
Or should it be specific for each provider and defined in the AddXxx() method ?

[divega]

This makes sense to me. If we add this more general purpose extension I think we should discuss in detail how we make the file-based providers converge to it and how much compatibility we need to maintain.

I think it is possible to make the pattern more generic by changing some of the property types on https://github.com/aspnet/Configuration/blob/dev/src/Microsoft.Extensions.Configuration.FileExtensions/FileLoadExceptionContext.cs. Hopefully we can figure a way so that usability for file-based providers is not affected.

[divega]

cc @HaoK @glennc

[HaoK]

We can certainly push it into base, but then it needs to go onto then base ConfigurationProviderSource as well

[HaoK]

We'd probably also have to change the current ConfigurationProvider Load() to be non virtual and introduce another virtual load to automatically hook this up

[HaoK]

@divega do you want me to take a look at this for 2.0?

[divega]

@HaoK sounds good. cc @glennc