internal/fs: update error message when rename fallback fails

[ibrasho]

Would clarify errors like (from #651):

safe write of manifest and lock: second attempt failed: cannot rename /tmp/dep666066408/vendor to /home/ser/workspace/src/foo/vendor: read /tmp/dep666066408/vendor/github.com/prometheus/procfs/fixtures/self: is a directory

to become:

safe write of manifest and lock: rename fallback failed: cannot rename /tmp/dep666066408/vendor to /home/ser/workspace/src/foo/vendor: copying file failed: read /tmp/dep666066408/vendor/github.com/prometheus/procfs/fixtures/self: is a directory

[sdboyer]

All this multilayer error wrapping seems like a code smell to me, but i don't have a well-formed opinion about what we'd do instead, so... 😄