Adds script termination in case Agent is not installed for Agent Binary fetching

Author: LordHepipud

lib/core/icingaagent/getters/Get-IcingaAgentBinary.psm1

@@ -1,6 +1,10 @@
 function Get-IcingaAgentBinary()
 {
     $IcingaRootDir = Get-IcingaAgentRootDirectory;
+    if ([string]::IsNullOrEmpty($IcingaRootDir)) {
+        throw 'The Icinga Agent seems not to be installed';
+    }
+
     $IcingaBinary = (Join-Path -Path $IcingaRootDir -ChildPath '\sbin\icinga2.exe');
 
     if ((Test-Path $IcingaBinary) -eq $FALSE) {