Add more explicit console logging + minor fixes

* The checks with keywords such as "Should Not Be Empty", "Should
  Be Equal As"... should have a more explicit message on fail, so that
  the console output shows a clearer failing reason.
* Other minor fixes included in the commit are:
	* The Sadis file comparison to None string.
	* The KUBECTL_CONFIG variable not properly set, altough
	  apparently unused.

Change-Id: I9305ba791dd43c9088a7bcbcc9af568ae127f60f
diff --git a/libraries/bbf_adapter_utilities.robot b/libraries/bbf_adapter_utilities.robot
index 2c25099..e169373 100644
--- a/libraries/bbf_adapter_utilities.robot
+++ b/libraries/bbf_adapter_utilities.robot
@@ -503,7 +503,7 @@
     [Arguments]      ${device_serial_number}    ${isONU}
     ${cmd}=     Catenate    voltctl -c ${VOLTCTL_CONFIG} device list | grep ${device_serial_number}
     ${rc}    ${rest}=    Run and Return Rc and Output    ${cmd}
-    Should Not Be Empty    ${rest}
+    Should Not Be Empty    ${rest}      Could not find device in VOLTHA
     Run Keyword If   ${isONU}
     ...     Correct Representation check ONU Voltha-IETF    ${rest}
     ...     ELSE