Adapt gdown to be used as python library

[Skitionek]

Summary

• Exceptions are printed only on cli (otherwise simply raised)
• Allows for custom progress callbacks (which defaults to old behaviour)
• stderr stands for "standard error" - progress messages should be printed to stdin

[wkentaro]

The change to Exception looks good, but I'm not sure about others. If you split PRs, we can merge some of them earlier.

[wkentaro]

Regarding stderr vs stdout, this is the reason why we keep stderr for the log messages.

[Skitionek]

Regarding stderr vs stdout, this is the reason why we keep stderr for the log messages.

Interesting, yet putting flag --quiet should silence all progress prints from code.... Therefore it still can be used the way you mentioned.

[Skitionek]

The change to Exception looks good, but I'm not sure about others. If you split PRs, we can merge some of them earlier.

Moved Exceptions to separate PR: #246