Subcommand segfault if the "authors" attribute is missing from "project"

Cargo outputs `subcommand failed with signal: 11` for every subcommand if the "authors" attribute is missing. E.g. this segfault:

```
[project]
name = "segfault"
version = "0.0.1"
#authors = ["UberLambda <paolo.jovon@gmail.com>"] <- Omitting this segfaults subcommands

[[lib]]
...
```

A proper error message would be more user-friendly.
