Wrap URI.parse() call in try ... catch

[sryze]

When URI.parse() encounters an invalid URL it can throw a URIError, but the error is never caught and it pops up in the user's face (issue #173). So this patch simply puts the parse call in a try ... catch statement.

I don't know if the URI library is supposed to throw an error in this case, it may be an issue with vscode-uri.

Steps to reproduce the error:

1. Create a file named test.scss
2. Paste:

.test {
    background: url('<%=invalid%>');
}

3. Open the file in VS Code and observe an error notification:

If you are wondering why I would be using an invalid URL - it's not really invalid, the CSS file produced by the SCSS compiler in my project is included inline into a JSP page, and the url() actually contains a JSP echo tag inside (<%=some expression%>). This used to work in VS Code 1.36 and earlier, but started to throw an error in newer releases.

[aeschli]

Looks good, thanks @sryze !