Handle auth-source-search failures in open-network-stream

Handle auth-source-search failures in open-network-stream

If the user cancels the gpg decryption pop-up, auth-source-search
fails *and* epa pops up an error buffer.  Fix epa to allow suppressing
that, and ignore errors returned from auth-source-search.

* lisp/epa.el (epa-suppress-error-buffer): New defvar.  Bind non-nil
to stop epa popping up an error buffer.

* lisp/net/network-stream.el: require epa when byte-compiling.
(network-stream-certificate): ignore errors when calling
auth-source-search, and suppress the epa error buffer.