Visible TokenExchangeRequestValidator fail message

[EgorTarasovSecurrency]

Currently, TokenExchangeRequestValidator fail message is effectively lost since it is exchanged to the generic message

TokenExchange/src/Rsk.TokenExchange.IdentityServer4/TokenExchangeExtensionGrantValidator.cs

Line 70 in b999e77

context.Result = new GrantValidationResult(TokenRequestErrors.InvalidGrant, "Invalid subject token");

It would be nice to make the error description visible. Perhaps, something like that:

context.Result = new GrantValidationResult(TokenRequestErrors.InvalidGrant, $"Invalid subject token - {validationResult.ErrorDescription}"); 

If you will accept PRs on the change, please let me know