Bug #4857 : fixing photo technical error on save due to bad IPTC encoding values by catching unsupported encoding exception.
bug #4857 :  Remove the exception stacktrace when an UnsupportedEncodingException exception is catched when fetching the image metadata. Catch also this exception (without its stacktrace) when fetching explicitly the image metadata (in ImageHelper#setMetadata). By removing the stacktrace, the logs in trace.txt will be more readable

Conflicts:
	gallery/gallery-ejb/src/main/java/com/silverpeas/gallery/ImageHelper.java