Having a better error message for missing gpg key during package build

[kushaldas]

We now have support for reproducible tarball dynamically via #185. But, if the GPG key is missing on the system, it will provide
an error message saying not signed with KEYID.

Receiving objects: 100% (9312/9312), 6.98 MiB | 4.09 MiB/s, done.
Resolving deltas: 100% (6892/6892), done.
Failed to verify 0.2.1, not signed with 22245C81E3BAEB4138B36061310F561200F4AD77
make: *** [Makefile:9: securedrop-client] Error 2

But, the actual error is missing key, if we see not signed, that may mean an attack of some sort :)

How to reproduce?

• Delete the key gpg2 --delete-keys 22245C81E3BAEB4138B36061310F561200F4AD77
• Try to build securedrop-client make securedrop-client