DDC-3961: orm:validate-schema exits with exception instead of reporting ManyToMany mapping error

Jira issue originally created by user benjamin:

When using bi-directional many-to-many associations, and forgetting to use mappedBy and indexedBy, the error is not reported when validating the mapping (orm:validate-schema --skip-sync).

Example code:

    class Cuisine {
        /****
         * @ManyToMany(targetEntity="Dish")
         * @ORM\JoinTable(name="CuisineDish", ...)
         */
        protected $dishes;
    }

    class Dish {
        /****
         * @ORM\ManyToMany(targetEntity="Cuisine")
         * @ORM\JoinTable(name="CuisineDish", ...)
         */
        protected $cuisines;
    }

Furthermore, it triggers an exception when validating the sync with the database (orm:validate-schema [--skip-mapping]).

The exception is:

{quote} [Doctrine\DBAL\Schema\SchemaException]
The table with name 'mydb.CuisineDish' already exists.
{quote}

This exception should not be thrown, and its message is really confusing to the end user, as it suggests that Doctrine is attempting to create a table in the database, whereas this action is read-only.