Unhelpful error message on invalid file

[FiloSottile]

Discussed in #415

^{Originally posted by shepting May 5, 2022}
I was recently using age in an automated fashion for some CI machines and received the following message:

age: error: failed to read header: parsing age header: failed to read intro: EOF

Unfortunately, since the error message didn't specify either:

1. the filename of the invalid file or
2. the first line of the file it was parsing
   I spent 30 minutes investigating what was wrong with my private key on my machine without realizing it was the "encrypted" file that was failing to parse!

My command was something like cat ~/setup/id_rsa.encrypted | age --decrypt --identity ~/.age/ci.key --output ~/.ssh/id_rsa so specifying that it was STDIN that was failing could help. Even moreso if it gave the first line of STDIN that didn't look right.

[covert-encryption]

Would make sense to give different messages for errors reading identity files and data files, e.g. prepending text on the error messages depending on which context it comes from, while keeping the original message for detailed information on what went wrong.