Before this change, several of the potential errors during the process
of signing a package were skipped.
Crucially, `Close()`ing the ReadCloser from the gpg clearsigner is the call
which actually does the signing, and so has several points of failure
which are ignored; for example, if there's a problem with the format of
the key.
Also changes the error from messageBlock() to be propagated rather than
being swallowed, and adds a test for the case where a signer fails to
sign.
Signed-off-by: Ashley Davis <ashley.davis@jetstack.io>