Fix owner list package

[paulo-ferraz-oliveira]

There's two fixes to this:

1. if a package doesn't exist we don't crash, but instead output Error listing owners of package PACKAGE: Entity not found (404) (where PACKAGE is the queried one),
2. if a package's owner doesn't have his email listed we don't crash[1]

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[1] regarding this I don't know if 1. this is an Hex issue, that shouldn't return an empty e-mail field, an httpc issue (because the field is defined as nil) or a rebar3_hex issue, in which case the fix might be acceptable.

[paulo-ferraz-oliveira]

Most recent commit might just address both your comments at once. Lemme know.

[starbelly]

@paulo-ferraz-oliveira I believe they do. Build is failing for some reason (not circle-ci)

[paulo-ferraz-oliveira]

Lemme check this.

[paulo-ferraz-oliveira]

Thanks much.