Handle the case when revision is wrong/non-existant

[knl]

When running the following:

niv update terraform-provider-github -a rev=v2.21.0

I got:

Update terraform-provider-github
  Reading sources file
Could not read the output of 'nix-prefetch-url'. This is a bug. Please create a
ticket:

  https://github.com/nmattia/niv/issues/new

Thanks! I'll buy you a beer.
stdout:

stderr:
error: unable to download 'https://github.com/terraform-providers/terraform-provider-github/archive/v2.21.0.tar.gz': HTTP error 404


One or more packages failed to update:
terraform-provider-github: ExitFailure 1

ERROR: ExitFailure 1

The problem is that v2.21.0 doesn't exist (I made a typo).