Print better error msgs when drom fails to parse package name

[Karfroth]

This is for issue 117.

At this point Drom does not mention which package has the problem when it fails to parse name. This PR improves error msg by checking the package section's dir field and file existence

if dir field is None, error msg will say a package does not have name in drom.toml file.
If dir field is given but file cannot be found, the msg will say package.toml is missing
if dir field is given, then it will print name field is missing.

[zapashcanon]

Thanks !