Throw a better error to distinguish relative meta-schema references

[jviotti]

According to the JSON Schema specification (https://json-schema.org/draft/2020-12/json-schema-core#section-8.1.1-2), relative meta-schema references are not permitted. We obey this, but in such case (for example when running base_dialect()), we throw a standard SchemaResolutionError that can leave some users confused.

For example, consider this case:

{ "$schema": "../meta.json" }

And this meta-schema at the corresponding relative location:

{
  "$schema": "https://json-schema.org/draft/2020-12/schema",
  "$vocabulary": { "https://json-schema.org/draft/2020-12/vocab/core": true },
  "$dynamicAnchor": "meta",
  "$ref": "https://json-schema.org/draft/2020-12/meta/core"
}

When running this in the CLI, we would get:

error: Could not resolve the metaschema of the schema
  ../meta.json

This is likely because you forgot to import such schema using --resolve/-r

Instead, we should have a new SchemaRelativeMetaschemaResolutionError error (that subclasses SchemaResolutionError) to better react to this case and present information that the average user can actually understand.

Cases to consider seem to include:

• base_dialect() (for performance reasons, only parse the URI when we are about to throw SchemaResolutionError)
• dialect() (this one probably does not need to check anything and we can rely on its consumers to do so when appropriate)

[jviotti]

cc @Karan-Palan here is another rule (non auto fixable one) we should record for the future: $schema MUST be set to an absolute URI. Setting it to a relative one is invalid