Now emits warning if metadata can't be converted.

imageio/imageio-tiff/src/main/java/com/twelvemonkeys/imageio/plugins/tiff/TIFFImageWriter.java

@@ -866,7 +866,7 @@ public final class TIFFImageWriter extends ImageWriterBase {
                 }
             }
             catch (IIOInvalidTreeException e) {
-                // TODO: How to issue warning when warning requires imageIndex??? Use -1?
+                processWarningOccurred(sequenceIndex, "Could not convert image meta data: " + e.getMessage());
             }
 
             ifd = outData.getIFD();