Throw exception instead of silently failing if zip save/close fails (#54)

I saw a case before on my machine where closing the zip would fail to write out changes, but it would fail silently, so it wasn't obvious there was a problem there.
See more details here: https://xamarinhq.slack.com/archives/C03CEGRUW/p1588784048377800
This PR makes those errors explicit, throwing an exception on Close failures, so that the user will see there's a problem, that the ZIP/APK didn't actually get updated.
I'm honestly a little nervous about this change - it seems good conceptually but might there be cases in practice where it's better to fail silently?
I'm not sure, but I wanted to create a PR to get feedback from you all on that.
As for my machine, it was a failure to create temp file error (see Slack). The problem went away after I rebooted and I wasn't able to reproduce it again. Probably it was some file in use issue of some kind. This may or may not be a problem customers see in practice (of course, since it fails silently, we have less visibility into whether it is a problem in practice - making the error explicit would help with that at least).

Author: Bret Johnson

ZipArchive.cs

@@ -869,12 +869,16 @@ public void Close ()
 if(archive==IntPtr.Zero)
 return;
 
-Native.zip_close(archive);
-foreach(varsinsources){
-s.Dispose();
+try{
+if(Native.zip_close(archive)<0)
+throwGetErrorException();
+}finally{
+foreach(varsinsources){
+s.Dispose();
+}
+sources.Clear();
+archive=IntPtr.Zero;
 }
-sources.Clear();
-archive=IntPtr.Zero;
 }
 
 /// <summary>