generic: catch OAuth refresh token errors

In case we have an existing refresh token, but it has expired or been
revoked, we should gracefully fall back to performing an interactive
OAuth flow.

src/shared/Core/GenericHostProvider.cs

@@ -143,16 +143,26 @@ private async Task<ICredential> GetOAuthAccessToken(Uri remoteUri, string userNa
             ICredential refreshToken = Context.CredentialStore.Get(refreshService, userName);
             if (refreshToken != null)
             {
-                 var refreshResult = await client.GetTokenByRefreshTokenAsync(refreshToken.Password, CancellationToken.None);
+                try
+                {
+                    var refreshResult = await client.GetTokenByRefreshTokenAsync(refreshToken.Password, CancellationToken.None);
 
-                 // Store new refresh token if we have been given one
-                 if (!string.IsNullOrWhiteSpace(refreshResult.RefreshToken))
-                 {
-                     Context.CredentialStore.AddOrUpdate(refreshService, refreshToken.Account, refreshToken.Password);
-                 }
+                    // Store new refresh token if we have been given one
+                    if (!string.IsNullOrWhiteSpace(refreshResult.RefreshToken))
+                    {
+                        Context.CredentialStore.AddOrUpdate(refreshService, refreshToken.Account, refreshToken.Password);
+                    }
 
-                 // Return the new access token
-                 return new GitCredential(oauthUser,refreshResult.AccessToken);
+                    // Return the new access token
+                    return new GitCredential(oauthUser,refreshResult.AccessToken);
+                }
+                catch (OAuth2Exception ex)
+                {
+                    // Failed to use refresh token. It may have expired or been revoked.
+                    // Fall through to an interactive OAuth flow.
+                    Context.Trace.WriteLine("Failed to use refresh token.");
+                    Context.Trace.WriteException(ex);
+                }
             }
 
             // Determine which interactive OAuth mode to use. Start by checking for mode preference in config