cli: add NMC_RESULT_ERROR_NOT_FOUND and use as appropriate

This will allow callers to distinguish "no such connection" from
"connection failed to activate", etc.

https://bugzilla.redhat.com/show_bug.cgi?id=1022256

cli/src/nmcli.h

@@ -55,7 +55,10 @@ typedef enum {
 	NMC_RESULT_ERROR_NM_NOT_RUNNING = 8,
 
 	/* nmcli and NetworkManager versions mismatch */
-	NMC_RESULT_ERROR_VERSIONS_MISMATCH = 9
+	NMC_RESULT_ERROR_VERSIONS_MISMATCH = 9,
+
+	/* Connection/Device/AP not found */
+	NMC_RESULT_ERROR_NOT_FOUND = 10
 } NMCResultCode;
 
 typedef enum {