Signed-off-by: Krisztian Fekete <1246751+e3krisztian@users.noreply.github.com>

ValidationError had a single untyped property: .data, which hold
different objects in the XML and JSON cases, while they do have some
common properties.

The new properties introduced in this commit are
- .message: the error message text
- .path: pointer to the location of the error in the input

Signed-off-by: Krisztian Fekete <1246751+e3krisztian@users.noreply.github.com>

ValidationError.data and .message is provided by third party libraries,
and they can give a message of any length. E.g. jsonschema inserts its
input in all of its messages, which could be arbitrary big.

To be able to show these errors to the user, some pre-processing is
needed. The new method allows for squeezing these messages in a way,
that is least disruptive, and has special knowledge how to shorten
jsonschema messages.

It is definitely still a workaround, and ideally the libraries should
not yield unlimited messages.

Signed-off-by: Krisztian Fekete <1246751+e3krisztian@users.noreply.github.com>

There was only one validation method, that returned a single error (and
it was the first error (JSON) or the last one (XML)).

The new function `SchemabasedValidator.iterate_errors()` allows to
enumerate over all the validation errors.

Signed-off-by: Krisztian Fekete <1246751+e3krisztian@users.noreply.github.com>

…ll errors

Signed-off-by: Jan Kowalleck <jan.kowalleck@gmail.com>

Signed-off-by: Jan Kowalleck <jan.kowalleck@gmail.com>