fix: keep on processing after failing to read GIFs

Reading some GIF images caused EPUBCheck to crash with a
`java.lang.ArrayIndexOutOfBoundsException`. See #756.

This change expands the catch block to any `RuntimeException` to at
least catch the problem and keep on processing.

Further work needs to be done to handle #756 properly, and do not
reject said GIFs as corrupted.

src/main/java/com/adobe/epubcheck/bitmap/BitmapChecker.java

@@ -158,7 +158,7 @@ public ImageHeuristics getImageSizes(String imgFileName) throws
  report.message(MessageId.PKG_021, EPUBLocation.create(imgFileName));
  return null;
  }
- catch (IllegalArgumentException argex)
+ catch (RuntimeException argex)
  {
  report.message(MessageId.PKG_021, EPUBLocation.create(imgFileName));
  return null;