Symfony version(s) affected: 4.1

Description
Unresolved env vars are found in the dumped container when the Configuration class normalizes them by ignoring their values. I suppose an EnvParameterException should be thrown somewhere instead.

How to reproduce

• git clone https://github.com/nicolas-grekas/skeleton-unused-envs-bug
• look at the dumped container for the "foo" parameter's value, it currently will look like 'foo' => 'env_2268c5b83aab5515_FOO_4c8b0d563663fd466ddc8bdbfa49922d',