cfg: turn a handful of parameter parsing errors into fatal (#554)

For parameters that require two values (`width, height` or `x, y`), if
the string fails to parse, turn into runtime failure.

src/spice2x/overlay/windows/keypad.cpp

@@ -120,7 +120,7 @@ namespace overlay::windows {
             "Lightning Model cabinets (TDJ) do not have any keypads; they use the subscreen.\n\n"
             "Fullscreen mode: bind a key in Overlay tab, and press it in game to show the subscreen, "
             "then use your mouse to click. Page Up button is the default binding.\n\n"
-            "Windowed mode: look for the second window in the taskbar.\n\n"
+            "Windowed mode: look for the second window in the taskbar (or ALT+TAB).\n\n"
             "Windowed mode with -iidxnosub: bring up the subscreen overlay (default Page Up).\n\n"
             );