Obviously adding a explicit message when trying to use Unions but typeguard is not available.
Make typeguard a non-extra dependency. If marshmallow_dataclass drops support for older python versions (as suggested in PR #260) and the minimum version of marshmallow is raised from 3.13 to 3.18 this would allow to completely refrain from using extra dependencies.
Currently marshmallow_dataclass fails without an explicit error message when trying to use Unions but missing the
typeguard
dependency.