[csharp] Fix error loading library grpc_csharp_ext.*.dll on windows with non-ASCII encoding (#26762)

* [csharp net472] Fix error loading library grpc_csharp_ext.x64.dll on Windows with charset on non ANSI encoding. Example library path: C:\Program Files (x86)\тест\grpc_csharp_ext.x64.dll

* cleanup

* Change charset to Unicode

Co-authored-by: Jan Tattermusch <jtattermusch@google.com>

src/csharp/Grpc.Core/Internal/UnmanagedLibrary.cs

@@ -134,9 +134,19 @@ namespace Grpc.Core.Internal
         {
             if (PlatformApis.IsWindows)
             {
-                // TODO(jtattermusch): populate the error on Windows
                 errorMsg = null;
-                return Windows.LoadLibrary(libraryPath);
+                var handle = Windows.LoadLibrary(libraryPath);
+                if (handle == IntPtr.Zero)
+                {
+                    int win32Error = Marshal.GetLastWin32Error();
+                    errorMsg = $"LoadLibrary failed with error {win32Error}";
+                    // add extra info for the most common error ERROR_MOD_NOT_FOUND
+                    if (win32Error == 126)
+                    {
+                        errorMsg += ": The specified module could not be found.";
+                    }
+                }
+                return handle;
             }
             if (PlatformApis.IsLinux)
             {
@@ -179,13 +189,13 @@ namespace Grpc.Core.Internal
                 }
             }
             throw new FileNotFoundException(
-                String.Format("Error loading native library. Not found in any of the possible locations: {0}", 
+                String.Format("Error loading native library. Not found in any of the possible locations: {0}",
                     string.Join(",", libraryPathAlternatives)));
         }
 
         private static class Windows
         {
-            [DllImport("kernel32.dll")]
+            [DllImport("kernel32.dll", CharSet = CharSet.Unicode, SetLastError = true)]
             internal static extern IntPtr LoadLibrary(string filename);
 
             [DllImport("kernel32.dll")]