GnuPG at least 2.0.19 emits 'Bad session key' for a wrong passphrase
Check for the existence of a decrypted file instead of relying upon
Emacs's process filter

It seems that Emacs's process filter frequently drops some lines of
process's output. This patch works around this (long-standing?) Emacs
bug.
Merge pull request #21 from tadanagao/fix-gnupg2-things

Support GnuPG 2.0.19 and work around Emacs's process handling bug