KVSCode - validate if the klsp script is accesible

[radumereuta]

• give a proper error message if the klsp script is not in path
• verify that the LS actually starts and it is the right minimum version (someone pointed to the wrong script and it didn't work). It needs to be the installed version. Check all the dependencies because if you ignore these error messages at the startup you no longer know what to do.
• pop-up an error message and point to the documentation.
• make it clear in the readme that you need to have K installed.