Add error message dialog for pandoc error text

I tried to convert a document that contained a Markdown image link, and got the error

> The action “Run AppleScript” encountered an error.

Running the pandoc command gave me the following error:

> pandoc: Error producing PDF from TeX source.
> ! Package pdftex.def Error: File `http://oli.jp/img/logo.png' not found.
> See the pdftex.def package documentation for explanation.
> Type  H <return>  for immediate help.
>  ...
> l.263 ...cludegraphics{http://oli.jp/img/logo.png}

It would be nice to display this error, so I added a `try` wrapper and a basic error alert dialog.

CAVEAT: While it seems to work for me, I have no idea what I’m doing. _Caveat emptor — let the repo owner beware_ ;)