storage: improve error on missing output file

[buchanae]

The task outputs:

  "outputs": [
    {
      "name": "out",
      "url": "file:///mnt/funnel/dne/dne.txt",
      "path": "/opt/funnel/outputs/mnt/funnel/dne/dne.txt"
    }
  ],

The error log:

level | error
msg | Upload failed
error | failed to eval symlinks: lstat /mnt/exa/funnel/funnel-work-dir/b9no2ofntf6mbcjc5pk0/opt/funnel/outputs/mnt/funnel/dne/dne.txt: no such file or directory
url | file:///mnt/funnel/dne/dne.txt

The path /mnt/exa/funnel/funnel-work-dir is a VM host path and internal detail that isn't known by the user. Also, lstat isn't known by the user either.

We can probably improve this to something like Missing output file: /opt/funnel/outputs/mnt/funnel/dne/dne.txt

[adamstruck]

#552 addresses this for swift only