Show error message when gpg is not found

[rwstauner]

fixes #61

[Stratus3D]

Thanks @rwstauner ! Can you explain to me what the colon is for? I'm not familiar with the syntax. Is it equivalent to || true?

[rwstauner]

Essentially yes. It's a builtin command that does nothing.
From the "shell builtin commands" section of the bash man page:

       : [arguments]
              No effect; the command does nothing beyond expanding arguments and
              performing any  specified  redirections.   A zero exit code is returned.