lib/gpg: Only show gpg-connect-agent stderr on failures

lib/gpg: Only show gpg-connect-agent stderr on failures

When listing GPG keys, the temporary GPG homedir will be constructed by
simply copying the remote's trusted keys to the pubring.gpg file. In
that case, no GPG operations spawning gpg-agent will be run. When
gpg-connect-agent is run to cleanup the homedir, it will helpfully print
on stderr that it's starting gpg-agent like so:

gpg-connect-agent: no running gpg-agent - starting '/usr/bin/gpg-agent'
gpg-connect-agent: waiting for the agent to come up ... (5s)
gpg-connect-agent: connection to agent established

Send gpg-connect-agent's stderr to a pipe and only send it to the
application's stderr if an error was encountered.

Fixes: #1907
Closes: #1908
Approved by: cgwalters