Report path to missing internal key

[pjungwir]

Thank you for continuing to maintain this project!

I had a repo I started with git-crypt in 2018, and after some OS upgrades git status started to fail with this cryptic error:

git-crypt: Error: Unable to open key file - have you unlocked/initialized this repository yet?
error: external filter '"git-crypt" clean' failed 1
error: external filter '"git-crypt" clean' failed             
fatal: bin/bin/foodb: clean filter 'git-crypt' failed  

My gpg key was still there, so I didn't know what the problem was. Finally I looked at the gpg-crypt source code, and there was already a TODO comment to report the path to the missing key. Here is a PR adding that.

Once I made this change, I could see that it was looking for .git/common/git-crypt/git-crypt/keys/default. Instead I had .git/git-crypt/keys/default. So after this my repo worked again!:

mkdir -p .git/common/git-crypt/git-crypt/keys/
cp -a .git/git-crypt/keys/default !$/