fetch error message could be more descriptive

I made a typo when adding a dependency to vendor/:

 $ gvt fetch golang.org/x/tools/loader
2016/05/01 22:45:32 command "fetch" failed: lstat /tmp/gvt-126032266/loader: no such file or directory
 $ gvt fetch golang.org/x/tools/go/loader
[...]

Only after I entered that temporary directory did I figure out that I was using loader instead of go/loader - for a second I thought something else was going wrong. Perhaps the error given by gvt could be more descriptive if the vcs repository was cloned, but the package in it doesn't exist.