macOS support; better error messages; `--tries`

macOS support

Thanks to @Xytime (q.v. #1), macOS support is now in shticker_book_unwritten. shticker_book_unwritten now supports all of the same platforms that TTR does!

Better error messages

Error messages that are emitted should be improved in terms of how much useful information they provide, and also in their wording to some extent.

--tries

Previously, attempting to download game files would be done for a maximum of 5 tries (meaning that 5 failures was just enough to finally give up). Now, using the --tries argument (or its shorter version, -t), this value can be set by the user. The default is still 5.

Also, this retrying mechanism is now extended to downloading of the patch manifest from TTR’s servers.