Can/should OpenIdConnectCachingSecurityTokenProvider be fault tolerant

Copied from https://github.com/AzureADQuickStarts/B2C-WebApp-WebAPI-OpenIDConnect-DotNet/issues/9#issuecomment-323183755 as @parakhj says the old sample will be deprecated...

Issue I had today was my API couldn't connect to Azure B2C, during startup which caused a yellow screen with the API.

```
[WebException: The remote server returned an error: (500) Internal Server Error.]
System.Net.HttpWebRequest.EndGetResponse(IAsyncResult asyncResult) +606
System.Net.WebClient.GetWebResponse(WebRequest request, IAsyncResult result) +19
System.Net.WebClient.DownloadBitsResponseCallback(IAsyncResult result) +91
System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) +92
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) +58
Microsoft.IdentityModel.Protocols.d__4.MoveNext() in c:\workspace\WilsonForDotNet45Release\src\Microsoft.IdentityModel.Protocol.Extensions\Configuration\GenericDocumentRetriever.cs:42

[IOException: Unable to get document from: https://login.microsoftonline.com/contoso.onmicrosoft.com/v2.0/.well-known/openid-configuration?p=B2C_1_StandardSignup]
Microsoft.IdentityModel.Protocols.d__4.MoveNext() in c:\workspace\WilsonForDotNet45Release\src\Microsoft.IdentityModel.Protocol.Extensions\Configuration\GenericDocumentRetriever.cs:48
System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) +92
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) +58
Microsoft.IdentityModel.Protocols.d__0.MoveNext() in c:\workspace\WilsonForDotNet45Release\src\Microsoft.IdentityModel.Protocol.Extensions\Configuration\OpenIdConnectConfigurationRetriever.cs:81
System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task) +92
System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task) +58
System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd(Task task) +25
Microsoft.IdentityModel.Protocols.d__3.MoveNext() in c:\workspace\WilsonForDotNet45Release\src\Microsoft.IdentityModel.Protocol.Extensions\Configuration\ConfigurationManager.cs:198
```
Given that we are making an RPC call which can fail, does it make sense for the policy acquisition not to fail/throw so that the API doesn't crash at startup, it just can't authenticate using OAuth. When the service comes back, it will then pick up the policies, something like...

```
private void RetrieveMetadata()
{
_synclock.EnterWriteLock();
try
{
OpenIdConnectConfiguration config = Task.Run(_configManager.GetConfigurationAsync).Result;
_issuer = config.Issuer;
_tokens = config.SigningTokens;
}
catch (AggregateException ex)
{
// Unwrap and log
}
catch (Exception ex)
{
// Log failure
}
finally
{
_synclock.ExitWriteLock();
}
}
```
What I then get if the front-end can talk to B2C are
DX10500: Signature validation failed. Unable to resolve SecurityKeyIdentifier: 'SecurityKeyIdentifier errors as the issuer/token are null in the API.

What I don't know is if this is appropriate, i.e. JwtFormat keep calling the IIssuerSecurityTokenProvider, or am I abusing the interface.