Error message should include filename when open fails with "no such file or directory"

This is a feature request for more informative error messages. When a file cannot be opened, please include the actual filename in the error message.

Here is a typical example, ... When launching a CFSSL server using a command like this:

cfssl serve -ca-key root-ca-key.pem -ca root-ca.pem -config config_root_ca.json

it is common to encounter errors like this:

2018/12/05 00:17:55 [WARNING] couldn't initialize signer: open : no such file or directory
2018/12/05 00:38:22 [WARNING] couldn't initialize ocsp signer: open : no such file or directory

Trying to resolve these errors is made unnecessarily difficult because the error messages neglect to include the actual filename that could not be opened.