Properly check exit code of excludelist.sh in qmake

qmake is really making this overly complicated.

We should rather try to use this syntax, which is only available in
Qt 5.6 and higher, apparently:
https://doc.qt.io/qt-5/qmake-function-reference.html#system-replace

tools/linuxdeployqt/linuxdeployqt.pro

@@ -39,9 +39,17 @@ contains(DEFINES, EXCLUDELIST.*) {
     message("EXCLUDELIST specified, to use the most recent exclude list, please run qmake without EXCLUDELIST definition and with internet.")
 } else {
     message("Creating exclude list.")
+
+    # check whether command _would_ run successfully
+    EXCLUDELIST_GENERATION_WORKS = FALSE
+    system($$_PRO_FILE_PWD_/../excludelist.sh): EXCLUDELIST_GENERATION_WORKS = TRUE
+    isEqual(EXCLUDELIST_GENERATION_WORKS, FALSE) {
+        error("Generating excludelist failed")
+    }
+
     EXCLUDELIST = $$system($$_PRO_FILE_PWD_/../excludelist.sh)
     isEmpty(EXCLUDELIST) {
-        error("You must have internet to update EXCLUDELIST or define it in qmake.")
+        error("Generated excludelist is empty")
     }
     DEFINES += EXCLUDELIST=\""$$EXCLUDELIST"\"
 }