Convert some warnings to Error in zlib extension

[Girgias]

No description provided.

[Girgias]

If there are no objections I'll rebase and merge this next week.

[ghost]

Shouldn't this line also be changed to throw? I was only scrolling to that function because I was interested in seeing whether partial data fed to a zlib inflate context would throw now with this PR.

[nikic]

Shouldn't this line also be changed to throw? I was only scrolling to that function because I was interested in seeing whether partial data fed to a zlib inflate context would throw now with this PR.

No, this should not throw Error, as it's an expected condition.

[nikic]

LG. Would be great if you could also change the error messages to capitalize the first character.