Print library warnings to stderr with `warn` (not `puts`).#416

Merged

jdleesmiller merged 3 commits intorubyzip:masterfrom

hainesr:warnings

Oct 19, 2019

Commits on Oct 11, 2019

Use `warn` instead of `puts` for messages from the library code.
hainesr
committed

Commits on Oct 12, 2019

Make warning messages consistent.
hainesr
committed

Commits on Oct 13, 2019

Update tests to check error messages.
hainesr
committed