Instead of passing the error codes out of the function, just handle the
errors directly as they happen, and fail gracefully if something goes
wrong instead of continuing.
Whenever you would press Alt+Enter, or Alt+F, or on macOS Command+Enter,
or on macOS Command+F, or F11, the game would print this useless error
message to console, every single time: "Error: failed: " and it would
concatenate SDL_GetError() after it, but most of the time SDL_GetError()
is blank, so it would print just that.
Instead, what the fullscreen shortcut will now do is check the result of
the relevant SDL functions, BEFORE it decides to print an error message.
And when it DOES print an error message, it will be less vague and will
say instead "Error: toggling fullscreen failed: <output of
SDL_GetError()>".
This means Screen::ResizeScreen() and Screen::toggleFullScreen() are now
int-returning functions. Ideally, every function interfacing with SDL
would return an error code, but that's too much for this simple patch.
Additionally, I took the opportunity to clean up the surrounding
formatting of the code a bit, most notably dedenting the
keypress-clearing stuff by one tab level, converting the
shortcut-handling code to spaces, and removing commented-out code.
