[BUG] importing `dm_control` environments when the library is not installed does not display the right error message

[vmoens]

Describe the bug

We allow users to load the file envs/libs/dm_control.py even if dm-control is not installes. When creating an object in this situation, the error message now is that the environment configuration cannot be found. An ImportError should be raised instead (after a check for _has_dmc has been made).