Expose swagger parser errors as violations

[tkrop]

Zally is missing to expose the swagger-parser errors and silently succeeding instead of failing with violations. The swagger-parser creates a number of failure messages when discovering syntax errors. These are silently ignored, when an API is linted (see DefaultContextFactory.kt#L111). Instead we should convert them into violations of rule #101 (see also DefaultContextFactory.kt#L108).

This is a bit tricky since we need to refactor the error model used distinguish between OpenAPI 3.0 and Swagger 2.0.