Verify that the library files exist

Hi,

Regarding patch from comment 8, when looking for dependencies:

    if (array_key_exists('dependencies', $library)) {
      foreach ($library['dependencies'] as $dependency) {
        $parts = explode('/', $dependency, 2);
        $dependencyLibrary = $this->getLibraryByName($parts[0], $parts[1]);
        $valid = $this->isLibraryValid($dependencyLibrary);
      }
    }

I think it should return FALSE as soon as a dependency is not satisfied.

And also before the dependencies, as soon as file is not found it should return FALSE.

MR created from patch 8 + my suggestion.

Test not copied because a test file with the same name exists now in Core.