Better handling of missing elements

[colindean]

I've got a CI job that is creating a .netrc in a Make task using environment variables set at the CI level. It turns out that a typo caused the envvar containing the password to be wrong, so our make netrc wrote a line without a password.

The exception that libnetrc threw was unhelpful, simply saying "end of file" from here.

It'd be more user-friendly if the error said what it was looking for when it ran out of file in which to look for it.