validate.py should return an error if schema can't be found

[magnusbaeck]

Description

If examples/validate.py can't find a schema referenced in one of the example files this only results in a warning and a zero exit code rather than a non-zero exit code that one could reasonably expect.

Motivation

A non-existent schema version referenced in an example file is likely to be a real error and should be treated as such. I can't come up with a use case where you'd have example files that reference non-existent schema versions.

Exemplification

Changing the event version of a randomly chosen example file to something bogus could easily be overlooked (especially if #265 is implemented and we start trusting the validation results):

$ python examples/validate.py
Loaded 105 schemas.
Loaded 94 examples.
WARNING: Missing schema for aaaaaaaa-bbbb-5ccc-8ddd-eeeeeeeeeee0(EiffelActivityCanceledEvent) in examples/events/EiffelActivityCanceledEvent/simple.json.

===SUMMARY===
Bad schema files:         0
Bad example files:        0
Successful validations:   93
Failed validations:       0
Unchecked examples:       1
=============
$ echo $?
0

Benefits

Reduced risk of mistakes when updating the example files.

Possible Drawbacks

None? Making missing schemas a non-error appears to be a conscious decision but I don't understand that rationale behind it. If there's a reasonable use case for the current behavior we could add a --strict flag or similar, or the reverse by making the default behavior fatal and adding a --ignore-missing-schema flag.

[e-backmark-ericsson]

I agree. The examples should reference valid schemas.