add a comment for image parsing issue

Author: stevenolen

src/org/ohmage/domain/Image.java

@@ -634,6 +634,7 @@ public BufferedImage getBufferedImage() throws DomainException {
 								"The image contents are invalid.");
 					}
 				}
+				// catch IllegalArgumentException when image has error in exif data.
 				catch(IOException|IllegalArgumentException e) {
 					throw
 						new DomainException("The image could not be read.", e);