SecurityInterceptor does not consult the GRpcErrorHandler for auth scheme selection errors

[jGleitz]

For example, BearerTokenAuthSchemeSelector throws a OAuth2AuthenticationException if a bearer token is malformed. Since the error handler is not consulted, we cannot remap this exception. As a consequence, our service returns the code Unknown, although it should be Unauthorized.

[jGleitz]

We can currently work around this issue by building an interceptor that has higher precedence than the SecurityInterceptor. Still it would be easier if the GRpcErrorHandler could handle it.

[jvmlet]

@jGleitz ,I've refactored the test a bit