Consider BaseplateError exceptions with code=5xx an error for span accounting

[spladug]

We don't generally consider IDL-specified Thrift exceptions as span failures (since they're generally equivalent to a 4xx in HTTP terms -- they're part of a valid response rather than an indication the service failed). However, when we create BaseplateError exceptions with explicit 5xx error codes we should count those as service failures and set success=False in span accounting.