Fail Gracefully on Invalid YAML File Format

Problem/Motivation

If I try importing a file that is not in YAML format it causes a fatal error, because the parseContent method of the ContentLoader class does not check whether the file contents were parsed correctly before trying to load them.

Steps to reproduce

• Create a test file in the content directory with the correct naming, such as file1.content.yml, but instead of YAML syntax, put some other text in there such as PHP code.
• Load the file via YAML Content drush command as usual.
• See that there is a fatal error like
  

  Drupal\yaml_content\Event\ContentParsedEvent::__construct() must be of the type array, string given

Proposed resolution

Add logic to check if YAML parsing was successful. If not, display an error message and do not try to load the file.

Remaining tasks

User interface changes

API changes

Add logic to yaml_content/src/ContentLoader/ContentLoader.php, parseContent() method

Data model changes