我正在尝试在运行 10.7.4 的 MacBook Pro 2011 上安装 Xamarin。当我运行安装程序时，它首先尝试安装 Android SDK，但是过了一会儿它失败了并且错误报告指出：
Error downloading from Google server. Android SDK.
The operation has timed out.
Exception type: System.Net.WebException
Message: The operation has timed out.
at System.Net.WebConnectionStream.Read (System.Byte[] buffer, Int32 offset, Int32 size) [0x00000] in <filename unknown>:0
at Xamarin.Web.Installer.DownloadService.DoDownload (Int64 totalBytes, System.IO.Stream responseStream, System.IO.Stream outputStream, System.Object state) [0x00000] in <filename unknown>:0
at Xamarin.Web.Installer.Utilities.DownloadUrl (System.Net.HttpWebResponse response, System.String url, System.Action`4 doDownload, Boolean noCache, System.Object state) [0x00000] in <filename unknown>:0
Android SDK installation failed with more than one exception (Attempt #Android SDK)
Some installation errors are present.
Exception type: System.AggregateException
Message: Some installation errors are present.
Exception type: Xamarin.Web.Installer.InstallException
Message: Download failed.
at Xamarin.Web.Installer.Installer.BaseSoftwareItem.ValidateDownloadItem () [0x00000] in <filename unknown>:0
at Xamarin.Web.Installer.Installer.AndroidSoftwareItem.Install (UInt32 tryNumber) [0x00000] in <filename unknown>:0
at MacInstaller.WizardPageInstallationController.InstallationWorker () [0x00000] in <filename unknown>:0
Exception type: System.Net.WebException
Message: The operation has timed out.
at System.Net.WebConnectionStream.Read (System.Byte[] buffer, Int32 offset, Int32 size) [0x00000] in <filename unknown>:0
at Xamarin.Web.Installer.DownloadService.DoDownload (Int64 totalBytes, System.IO.Stream responseStream, System.IO.Stream outputStream, System.Object state) [0x00000] in <filename unknown>:0
at Xamarin.Web.Installer.Utilities.DownloadUrl (System.Net.HttpWebResponse response, System.String url, System.Action`4 doDownload, Boolean noCache, System.Object state) [0x00000] in <filename unknown>:0
Exception type: Xamarin.Web.Installer.InstallException
Message: Download failed.
at Xamarin.Web.Installer.Installer.BaseSoftwareItem.ValidateDownloadItem () [0x00000] in <filename unknown>:0
at Xamarin.Web.Installer.Installer.AndroidSoftwareItem.Install (UInt32 tryNumber) [0x00000] in <filename unknown>:0
at MacInstaller.WizardPageInstallationController.InstallationWorker () [0x00000] in <filename unknown>:0
Exception type: System.Net.WebException
Message: The operation has timed out.
at System.Net.WebConnectionStream.Read (System.Byte[] buffer, Int32 offset, Int32 size) [0x00000] in <filename unknown>:0
at Xamarin.Web.Installer.DownloadService.DoDownload (Int64 totalBytes, System.IO.Stream responseStream, System.IO.Stream outputStream, System.Object state) [0x00000] in <filename unknown>:0
at Xamarin.Web.Installer.Utilities.DownloadUrl (System.Net.HttpWebResponse response, System.String url, System.Action`4 doDownload, Boolean noCache, System.Object state) [0x00000] in <filename unknown>:0
Exception type: Xamarin.Web.Installer.InstallException
Message: Download failed.
at Xamarin.Web.Installer.Installer.BaseSoftwareItem.ValidateDownloadItem () [0x00000] in <filename unknown>:0
at Xamarin.Web.Installer.Installer.AndroidSoftwareItem.Install (UInt32 tryNumber) [0x00000] in <filename unknown>:0
at MacInstaller.WizardPageInstallationController.InstallationWorker () [0x00000] in <filename unknown>:0
Exception type: System.Net.WebException
Message: The operation has timed out.
at System.Net.WebConnectionStream.Read (System.Byte[] buffer, Int32 offset, Int32 size) [0x00000] in <filename unknown>:0
at Xamarin.Web.Installer.DownloadService.DoDownload (Int64 totalBytes, System.IO.Stream responseStream, System.IO.Stream outputStream, System.Object state) [0x00000] in <filename unknown>:0
at Xamarin.Web.Installer.Utilities.DownloadUrl (System.Net.HttpWebResponse response, System.String url, System.Action`4 doDownload, Boolean noCache, System.Object state) [0x00000] in <filename unknown>:0
怎么了？