improve error when package.toml file is not found

[joprice]

If a package.toml file does not exist (likely due to a typo), the following error is show:

Error: Missing field 'name' in package.toml

If the file does not exist, I would expect an error saying that it is missing before attempting to parse it. If it is present and missing the name field, it would be helpful to show the full path to the package.toml, to help narrow down which package is causing the problem (In my case, I added several new packages to drom.toml and wasn't sure which was causing the issue).