[xa-prep-tasks] Improve <DownloadUri/> behavior (#302)

The `<DownloadUri/>` Task had two problems:

1. It used `HttpClient.GetAsync()`, not `HttpClient.GetStreamAsync()`.
    It *looks* like a minor difference, but the former would
    apparently attempt to store the *entire* source URI contents
    *in memory*, which would fail if the URI contents are "big".

2. Exceptions leaving `Task.Execute()` are "uglier" than exceptions
    logged via `TaskLoggingHelper.LogErrorFromException()`.

Improve the `<DownloadUri/>` task so that
`HttpClient.GetStreamAsync()` is used, removing the "download to
in-memory" behavior, and use
`TaskLoggingHelper.LogErrorFromException()` so that error messages are
nicer.

Additionally, `xa-prep-tasks.targets`
*wasn't supposed to surface errors* from the `_DownloadBundle` target.
The bundle which is downloaded *may not exist*. The build shouldn't
fail if the specified URI can't be found.

What's odd is that this *used* to work for me, in that if the target
URI -- the bundle -- didn't exist, `<DownloadTask/>` wouldn't report
an error, and execution would continue.

I suspect, but have not tested or verified, that Azure had changed
it's handling here: previously, it would return an XML document
containing a "404" error, even when the URL didn't exist.
This is *still the case* now [^0], so I'm not sure why I was *ever*
getting an error in the first place! (Very odd...)

"Sanitize" the behavior here: since the `_DownloadBundle` task can be
expected to "fail" -- in that the target URI may not exist, even if
Azure/etc. doesn't give a 404 for it -- then the `<DownloadUri/>` task
invocation should contain a `ContinueOnError="True"` attribute so that
errors are ignored.

Additionally, there is no need for the `_ExtractBundle` target to
execute unless the downloaded file exists, so make `_ExtractBundle`
conditional on the file existing, and update the
`_DownloadAndExtractBundle` target so that it explicitly orders the
dependent targets, so that `_ExtractBundle` doesn't need to depend on
the `_DownloadBundle` target.

[^0]: For example:

    $ curl -o x.txt https://xamjenkinsartifact.blob.core.windows.net/xamarin-android/xamarin-android/bin/does-not-exist
    $ $ cat x.txt
    <?xml version="1.0" encoding="utf-8"?><Error><Code>BlobNotFound</Code><Message>The specified blob does not exist.
    RequestId:d54a8d4a-0001-007d-1a0e-415487000000
    Time:2016-11-17T20:12:32.8482654Z</Message></Error>

build-tools/xa-prep-tasks/Xamarin.Android.BuildTools.PrepTasks/DownloadUri.cs

@@ -62,13 +62,18 @@ namespace Xamarin.Android.BuildTools.PrepTasks {
 			var dn  = Path.GetFileName (destinationFile);
 			var tempPath    = Path.Combine (dp, "." + dn + ".download");
 
-			Log.LogMessage (MessageImportance.Low, $"Downloading `{uri}` to `{tempPath}`.");
-			using (var r = await client.GetAsync (uri))
-			using (var o = File.OpenWrite (tempPath)) {
-				await r.Content.CopyToAsync (o);
+			Log.LogMessage (MessageImportance.Normal, $"Downloading `{uri}` to `{tempPath}`.");
+			try {
+				using (var s = await client.GetStreamAsync (uri))
+				using (var o = File.OpenWrite (tempPath)) {
+					await s.CopyToAsync (o);
+				}
+				Log.LogMessage (MessageImportance.Low, $"mv '{tempPath}' '{destinationFile}'.");
+				File.Move (tempPath, destinationFile);
+			}
+			catch (Exception e) {
+				Log.LogErrorFromException (e);
 			}
-			Log.LogMessage (MessageImportance.Low, $"mv '{tempPath}' '{destinationFile}'.");
-			File.Move (tempPath, destinationFile);
 		}
 	}
 }

build-tools/xa-prep-tasks/xa-prep-tasks.targets

@@ -17,12 +17,13 @@
       Inputs=""
       Outputs="$(_BundlePath)">
     <DownloadUri
+        ContinueOnError="True"
         SourceUris="$(_AzureBaseUri)$(Configuration)/$(XABundleFileName)"
         DestinationFiles="$(_BundlePath)"
     />
   </Target>
   <Target Name="_ExtractBundle"
-      DependsOnTargets="_DownloadBundle"
+      Condition=" Exists('$(_BundlePath)') "
       Inputs="$(_BundlePath)"
       Outputs="$(OutputPath).extracted-$(XABundleFileName)">
     <SystemUnzip
@@ -38,6 +39,6 @@
   </Target>
   <Target Name="_DownloadAndExtractBundle"
       AfterTargets="Build"
-      DependsOnTargets="_ExtractBundle">
+      DependsOnTargets="_GetBundleOutputPath;_DownloadBundle;_ExtractBundle">
   </Target>
 </Project>