Remain on the previous editing mode when the HTML-source conversion fails

When you switch from WYSIWYG to Source, remain on WYSIWYG if the conversion fails. Same for the reverse operation. Also show a notification message to explain the problem.

You can reproduce the behaviour like this:

• Edit Sandbox.WebHome
• Open the browser developer tools (right click outside the editing area and choose Inspect Element)
• Using the Inspector (top left corner in Firefox), select (click on) the first anchor in the editing area ("XWiki Syntax"). The corresponding anchor (a) element should get selected and you can see the HTML code
• Double click on the "false|-|doc|-|XWiki.XWikiSyntax" (the value of the "data-reference" attribute) and replace it with something else (e.g. "foo"). Press Enter key to update the HTML code.
• Switch to source