Lack of error message if the validators directory is not accessible (i.e. missing +x flag)

[zah]

Steps to reproduce on Unix-like systems:

1. Import some keystores into Nimbus
2. Remove the executable flag from the validator directory
3. Launch Nimbus and observe that no validator keys are loaded and no error message is printed.

Expected output:

The client should provide feedback to the user by printing an error message, suggesting how the problem can be resolved.