gpg 2.1: no error message if gpg-agent or pinentry fail

[Patrick Brunschwig]

If pinentry can't be launched, then gpg 2.0 will report an error saying
something like "problem with gpg-agent". But if you try to decrypt a
message with gpg 2.1 and pinentry doesn't work, then there is absolutely
no error message. You simply see "missing key xyz" messages.

This makes it very difficult to help people debug their setup (which
happens quite frequently on Linux). I would therefore appreciate if
there would at least be some human-readable error message; and even
better a parse-able message.

-Patrick