git-debpush: introduce and call fail_check

git-debpush: introduce and call fail_check

Without this, passing --force to override an earlier sanity check
might cause the user to miss the output of a later sanity check, which
would never get run.

We don't want to have multiple forcing options to override different
sanity checks, as that is too much complexity for a script like this.

So always run all checks, and error out afterwards if at least one of
them failed.

Signed-off-by: Sean Whitton <spwhitton@spwhitton.name>