Catch fs.watch exceptions

[campersau]

• Files / directories might get deleted between is.directory / is.file / fs.readdir and the actual fs.watch call.
• electron monkey patches the fs API to transparently support files contained in asar archives but fs.watch does not work for them. (See Electron version struggles with app.asar FredrikNoren/ungit#1523)

To test this I relied on the fact the the options are getting copied with Object.assign({}, options, {...}) so that I could delete the directory before fs.watch is called but after the is.directory / is.file checks.

Fixes #108

[yuanchuan]

Thank you