Improve error message for unsupported kubeconfig encoding

[omniproc]

Currently when a kubeconfig file is loaded that has a unsupported encoding, e.g. Windows Powershell's "Out-File" default UTF-16 LE BOM, Headlamp will return the very vague error message "Load a valid kubeconfig".
This makes it hard to understand what's wrong with the file given that the syntax of the file itself will be perfectly fine.

Headlamp should either support different encodings or at very least report back that an encoding error occurred.

Steps to reproduce:

1. Take a working kubeconfig my-working-kubeconfig.yaml and load it in Headlamp making sure it is accepted as valid kubeconfig
2. Run the Powershell snippet Get-Content my-working-kubeconfig.yaml -Raw | Out-File my-failing-kubeconfig.yaml
3. Try to load my-failing-kubeconfig.yaml