Fix Error 500 if Pygments can not hilight a code block in the Readme

When browsing a project file tree and there is a Readme file in the repository in some cases the Pygments syntaxhilighter can not process some code blocks and throws an exception (when python interpreter is not called python2). So I included a simple safeguard around the syntax hilighter to fall back to a standard 'pre'-block if Pygments raises an exception. So it renders the page at least not throwing a "500 Internal Server Error".

I know about https://github.com/gitlabhq/gitlabhq/issues/2214#issuecomment-11137058 but i think fixing this such a way it falls back to not hilighted simple code blocks could be a better option than to explode and throw a 500.