Fixing file copy issue

Fixes https://github.com/modcloth/bob/issues/18

The issue appears to have been in `bob.go` - the `bob.CleanWorkdir()` and `bob.Setup()` functions both return errors, but we weren't capturing those errors.  The result was that sometimes, copying files from the repo dir to the workdir would fail on certain files, but because the error wasn't getting checked, the flow would continue.  

**Why did the copy fail?** In one case, the file `.env` existed in the repo dir as a symlink to `.example.env`, which didn't exist.  Therefore, when bob tried to open the source file for the copy, it would fail.  I cannot state with certainty that this is the only case that would cause this bug to manifest.

**Why did that cause the build to fail *intermittently*?** Whether or not it failed was determined by the order in which the files got added.  Basically, whenever bob attempted to copy the "bad" file (`.env` in this case) all file copying would stop.  If that was the last file, as long as it wasn't explicitly added by a docker `ADD` command, the build would succeed.  However, if bob tried to copy the "bad" file first, then other important files wouldn't get copied over (i.e. the `Dockerfile`), and the build would fail.