Fix exception message for invalid custom string plugin (#4979)
Fix exception message for invalid custom string plugin (#4979)

* Fix exception error from generator to list of valid names

* Update pytorch_lightning/plugins/plugin_connector.py

Co-authored-by: Rohit Gupta <cb3c4569854e7764e66b4cfd98eab60a0e5fb067@gmail.com>

Co-authored-by: Rohit Gupta <cb3c4569854e7764e66b4cfd98eab60a0e5fb067@gmail.com>
Co-authored-by: Jirka Borovec <b903e28b3fb12ead55c25f3a7816376be3bd063c@users.noreply.github.com>
williamFalcon/pytorch-lightning,williamFalcon/pytorch-lightning