add fine-grained error messages to propagation.cpp

[dgoffredo]

The goal of these changes is to make this nginx error

2021/06/22 14:13:09 [error] 40#40: *53
failed to extract an opentracing span context from request 00005629525E3D70:
opentracing: SpanContext data corrupted in Extract carrier

look instead like

2021/06/22 14:13:09 [error] 40#40: *53
failed to extract an opentracing span context from request 00005629525E3D70:
opentracing: span has a parent ID but does not have a trace ID, unable to deserialize SpanContext

(though for this particular example the new code actually would not produce an error).

As part of these changes, I remove specific mention of error codes from the unit test. Instead, the tests only check that some error is returned.