Fixed error message in QAA when pipeline fails to execute (/status returns FAILURE)

[orviz]

After a failed pipeline the web does not permanently notify the user (with a fixed message) about this fact.

[orviz]

The same happens when a specific error code is returned from the API. For instance:

HTTP query: /assessment/<pipeline_id>/output
HTTP error code: 422
HTTP response:
Could not find report2sqaaas validator plugin <is_semver> (found: {'bandit': <class 'report2sqaaas_plugins_bandit.main.BanditValidator'>, 'boolean': <class 'report2sqaaas_plugins_boolean.main.BooleanValidator'>, 'cff': <class 'report2sqaaas_plugins_cff.main.CFFConvertValidator'>, 'fairEva': <class 'report2sqaaas_plugins_fairEva.main.fairEva'>, 'find_doc_files': <class 'report2sqaaas_plugins_find_doc_files.main.FindDocFilesValidator'>, 'flake8': <class 'report2sqaaas_plugins_flake8.main.Flake8Validator'>, 'json_not_empty': <class 'report2sqaaas_plugins_json_not_empty.main.JsonNotEmptyValidator'>, 'licensee': <class 'report2sqaaas_plugins_licensee.main.LicenseeValidator'>, 'markdownlint': <class 'report2sqaaas_plugins_markdownlint.main.MarkdownLintValidator'>})