Use levelDirError for graphics errors too

This will actually do several things:

(1) Make the tile size checks apply to the appropriate graphics files
    once again.
(2) Make the game print a fallback error message if the error message
    hasn't been set on the levelDirError error screen.
(3) Use levelDirError for graphics errors too.
(4) Make the error message for tile size checks failing specify both
    width and height, not just a square dimension.
(5) Make the error messages mentioned above translatable.

It turns out that (1) didn't happen after #923 was merged, since #923
removed needing to process a tilesheet into a vector of surfaces for all
graphics files except sprites.png and flipsprites.png. Thus, the game
ended up only checking the correct tile sizes for those files only.

In the process of fixing this, I also got rid of the PROCESS_TILESHEET
macros and turned them into two different functions: One to make the
array, and one to check the tile size of the tilesheet.

I also did (2) just in case FILESYSTEM_levelDirHasError() returns false
even though we know we have an error.

And (3) is needed so things are unified and we have one user-facing
error message system when users load levels. To facilitate this, I
removed the title string, since it's really not needed.

Unfortunately, (1) doesn't apply to font.png again, but that's because
of the new font stuff and I'm not sure what Dav999 has in store for
error checking. But that's also why I did (4), because it looks like
tile sizes in font.png files can be different (i.e. non-square).

desktop_version/lang/de/strings.xml

@@ -758,9 +758,11 @@ Du hast das Geheimlabor gefunden!" explanation="" max="34*4"/>
     <string english="Last 1 minute on the Super Gravitron" translation="Überstehe 1 Minute im Super Gravitron" explanation="achievement/trophy description" max="38*2"/>
     <string english="MASTER OF THE UNIVERSE" translation="HERRSCHER DES UNIVERSUMS" explanation="achievement/trophy title - no death mode complete" max="38*2"/>
     <string english="Complete the game in no death mode" translation="Beende das Spiel im Keine-Tode-Modus" explanation="achievement/trophy description" max="38*2"/>
+    <string english="Something went wrong, but we forgot to print an error message." translation="" explanation="the message that is printed in case the game detects an error but there&apos;s no error message set" max="38*6"/>
     <string english="Could not mount {path}: real directory doesn&apos;t exist" translation="Konnte {Pfad} nicht mounten: echtes Verzeichnis existiert nicht" explanation="mount: link/attach a directory (folder) in the filesystem into the game&apos;s filesystem so we can access it" max="38*6"/>
     <string english="Level {path} not found" translation="" explanation="" max="38*6"/>
     <string english="Error parsing {path}: {error}" translation="" explanation="we tried to parse the level file, but failed" max="38*6"/>
+    <string english="{filename} dimensions not exact multiples of {width} by {height}!" translation="" explanation="filename is something like tiles.png, tiles2.png, etc. and width/height are something like 8, 32, etc.; this is used if the dimensions of a graphics file aren&apos;t an exact multiple of the given size (e.g. 8x8, 32x32, etc.)" max="38*6"/>
     <string english="ERROR: Could not write to language folder! Make sure there is no &quot;lang&quot; folder next to the regular saves." translation="FEHLER: Konnte nicht in den Sprachordner schreiben! Stelle sicher, dass es keinen &quot;lang&quot;-Ordner neben den regulären Spielständen gibt." explanation="" max="38*5"/>
     <string english="" translation="" explanation=""/>
     <string english="" translation="" explanation=""/>