Inaccurate error message caused by lack of execute permission on folder containing keystore

[SeaMonkey82]

Was helping someone perform a voluntary exit and they were running into this error.

FAT 2023-06-13 02:17:08.901+12:00 Incorrect validator index, key or keystore path specified value=/var/lib/nimbus/validators/<VALIDATOR KEY> reason="Incorrect validator identifier value"

The real problem was that while the user executing the exit had read permissions for the keystore file, they did not have execute permissions on the directory containing the keystore file. I think the error messaging could be improved here.