Raise a clear warning message on startup when a snippets JSON file is invalid

[doronbehar]

I'm trying to write new snippets for personal use, and I noticed that if I have a mistake in my JSON file, no snippets are loaded from there, and I get this weird error:

{'throwpint': 'function vsnip#available[2]..vsnip#expandable[1]...te, line 5', 'exception': 'Vim(let):E474: Trailing comma: }^@}'}

Where the snippet file has indeed a trailing comma. Perhaps this message could be improved.