No error message is provided when there is a Network Failure

Reproduction steps

Use libgit2sharp in a bad network place

Expected behavior

Throw an exception that explicitly mentions network failure

Probably it's a good idea to handle situations where wifi signal is not strong or network failure. We could also handle this gracefully.

Actual behavior

Throws LibGit2Sharp.LibGit2SharpException saying

No error message has been provided by the native library

Stack trace,

Unhandled exception: System.Reflection.TargetInvocationException: Exception has been thrown by the target of an invocation.
 ---> LibGit2Sharp.LibGit2SharpException: No error message has been provided by the native library
   at LibGit2Sharp.Core.Ensure.HandleError(Int32 result)
   at LibGit2Sharp.Core.Ensure.ZeroResult(Int32 result)
   at LibGit2Sharp.Core.Proxy.git_remote_fetch(RemoteHandle remote, IEnumerable`1 refSpecs, GitFetchOptions fetchOptions, String logMessage)
   at LibGit2Sharp.Commands.Fetch(Repository repository, String remote, IEnumerable`1 refspecs, FetchOptions options, String logMessage)
   at LibGit2Sharp.Commands.Pull(Repository repository, Signature merger, PullOptions options)
   at GitUtil.Program.Main(String repoPath, String action, String filePath) in D:\git_ws\GitUtil\Program.cs:line 84
   --- End of inner exception stack trace ---
   at System.RuntimeMethodHandle.InvokeMethod(Object target, Span`1& arguments, Signature sig, Boolean constructor, Boolean wrapExceptions)
   at System.Reflection.RuntimeMethodInfo.Invoke(Object obj, BindingFlags invokeAttr, Binder binder, Object[] parameters, CultureInfo culture)
   at System.Reflection.MethodBase.Invoke(Object obj, Object[] parameters)
   at System.CommandLine.Invocation.ModelBindingCommandHandler.InvokeAsync(InvocationContext context)
   at System.CommandLine.Invocation.InvocationPipeline.<>c__DisplayClass4_0.<<BuildInvocationChain>b__0>d.MoveNext()
--- End of stack trace from previous location ---

Version of LibGit2Sharp (release number or SHA1)

0.27.0-preview-0158 and earlier

Operating system(s) tested; .NET runtime tested

Win 10, .Net SDK 6.0.101

从 Core.Ensure.HandleError 开始，沿着经过 Proxy.git_remote_fetch、Commands.Fetch 和 Commands.Pull 的错误路径进行跟踪。复现 issue 中描述的网络异常时的 fetch，并检查 native 错误是如何呈现的。完成的标准是：网络故障会产生一个带有明确且有用的网络相关消息的 exception，而不是 native library 的空消息。