if a directory is passed for a datasource.file millstone leaves behind symlink after failing

[springmeyer]

Layer Downloads in this case is not a file but a directory. We should avoid a symlink being made since this will/should fail later on:

[node] [millstone] processing layer 'downloads'
[node] [millstone] linking '/Users/dane/Documents/MapBox/project/a-buffer/layers/downloads' -> '/Users/dane/Downloads'
[node] [tilemill] problem loading datasource Error: Could not detect datasource type for: '/Users/dane/Documents/MapBox/project/a-buffer/layers/downloads'
[node]     at Function.resolved.srs (/Users/dane/projects/tilemill/node_modules/millstone/lib/millstone.js:833:41)
[node]     at next (/Users/dane/projects/tilemill/node_modules/millstone/node_modules/step/lib/step.js:51:23)
[node]     at /Users/dane/projects/tilemill/node_modules/millstone/lib/millstone.js:288:47
[node]     at fs.js:117:20
[node]     at /Users/dane/projects/tilemill/node_modules/npm/node_modules/graceful-fs/graceful-fs.js:53:5
[node]     at /Users/dane/projects/tilemill/node_modules/npm/node_modules/graceful-fs/graceful-fs.js:62:5
[node]     at /Users/dane/projects/tilemill/node_modules/glob/node_modules/graceful-fs/graceful-fs.js:53:5
[node]     at /Users/dane/projects/tilemill/node_modules/glob/node_modules/graceful-fs/graceful-fs.js:62:5
[node]     at Object.oncomplete (fs.js:297:15)