[libcalamares] Make running commands less chatty

If there's no output, don't mention it; don't mention failure modes
if the command was successful.

src/libcalamares/utils/CalamaresUtilsSystem.cpp

@@ -201,12 +201,29 @@ System::runCommand( System::RunLocation location,
     }
 
     auto r = process.exitCode();
-    cDebug() << Logger::SubEntry << "Finished. Exit code:" << r;
     bool showDebug = ( !Calamares::Settings::instance() ) || ( Calamares::Settings::instance()->debugMode() );
-    if ( ( r != 0 ) || showDebug )
+    if ( r == 0 )
     {
-        cDebug() << Logger::SubEntry << "Target cmd:" << RedactedList( args ) << "output:\n"
-                 << Logger::NoQuote {} << output;
+        if ( showDebug && !output.isEmpty() )
+        {
+            cDebug() << Logger::SubEntry << "Finished. Exit code:" << r << "output:\n" << Logger::NoQuote {} << output;
+        }
+        else
+        {
+            cDebug() << Logger::SubEntry << "Finished. Exit code:" << r;
+        }
+    }
+    else  // if ( r != 0 )
+    {
+        if ( !output.isEmpty() )
+        {
+            cDebug() << Logger::SubEntry << "Target cmd:" << RedactedList( args ) << "Exit code:" << r << "output:\n"
+                     << Logger::NoQuote {} << output;
+        }
+        else
+        {
+            cDebug() << Logger::SubEntry << "Target cmd:" << RedactedList( args ) << "Exit code:" << r << "(no output)";
+        }
     }
     return ProcessResult( r, output );
 }