Propagate errno for proper error messages

[nh2]

I'm using libE57Format through pye57 and noticed that E57_ERROR_OPEN_FAILED is raised for any error, including standard cases such as:

• no such file or directory
• permission denied
• too many open files

Most software can point out these error reasons by consulting errno (e.g. via strerror()), or at least guaranteeing in the API docs to not mess with errno so that the caller can inspect errno themselves.

It would be great if libE57Format could do one of these so that users can get much better error messages along with E57_ERROR_OPEN_FAILED.

[asmaloney]

I'm not sure I can guarantee the state of errno, but what I can do is provide the errno & string in the exception context.

So instead of:

result=-1 fileName=./no-path/foo.e57 flags=1538 mode=438

We'd have:

errno=2 error='No such file or directory' fileName=./no-path/foo.e57 flags=1538 mode=438

I don't know what pye57 does with the exception context, but would this solve your issue?

[nh2]

Yes, that would solve the issue!