fixes a problem with the --savetiddler command, 

it the tiddler doesn't exist and adds a propper error message

to reproduce 
```
node $(TIDDLYWIKI5_DIR)/tiddlywiki.js \
	editions \
	--verbose \
	--rendertiddler $:/core/save/all $(DEV_DIR)/index.html text/plain \
	--savetiddler $:/XXXXXXXXXXxxfavicon.ico $(DEV_DIR)/favicon.ico \
	|| exit 1
```
The XXXXfavicon tiddler will cause a stack trace error, which doesn't help. I did add a check and a propper message, that tells the user, what to do .