RFE: make fetch.snap errors more informative

[sabaini]

Upon a fault, the snap_*() functions which are utilizing the _snap_exec() will always return the same error regardless of underlying cause.

This is exacerbated by the fact that the errcode from the snap binary seems to be the same for all error types.

To be able to handle different error causes appropriately it would be helpful if the snap_*() functions returned different errors for different causes, or maybe just the snap error message along with the error code

[sabaini]

Also cf. https://bugs.launchpad.net/bugs/1905043 for getting more informative errcodes from snap