ROX-16728: On validation show concrete error

[janisz]

When user wan to validate configuration currently they will get generic message and stack trace in details.
This PR

• alters the message based on root cause.

• fixes page crash (unhandled exception) when user input invalid URL

• wrong answer from central (maybe it's not central at all or token is wrong)
  

• port is not listening
  

• wrong certificate
  

• unknown host
  

[rukletsov]

Looks good, thanks. I left a few minor comments. Also not sure you want to follow the seemingly established pattern here where error messages are defined in Messages.properties?

[dhaus67]

module the existing comments left.

[janisz]

Looks good, thanks. I left a few minor comments. Also not sure you want to follow the seemingly established pattern here where error messages are defined in Messages.properties?

@rukletsov I think we should remove Messages.properties. It's useful if we would like to translate the plugin but we never have such request so we move some strings to properties adding another moving element that we need to maintain (properties are handled by a generated helper).