Remove unnecessary details from error object

[Ben Sayers]

There are two situations causing confusion:

• When the error comes from the swagger-validation currently the pactDetails object is displayed in the result. Remove this, it is not needed.
• When the error comes from the pact-validation currently the swaggerDetails object is displayed in the result. Remove this, it is not needed.

[Ben Sayers]

feat: only output mockDetails and specDetails when they relevant

Close #56

→ <<cset 8486bf85b494>>