Give actionable error when base-libs download fails (#1332)

When the Unity base-libraries download fails (commonly a 404 for a
Unity version not yet hosted on unity.bepinex.dev), the uncaught
HttpRequestException surfaced only as a cryptic "Failed to generate
Il2Cpp interop assemblies" and the game would not launch -- even
though a local fallback (the unity-libs directory) already exists.

Catch the download failure and throw a clear message naming the Unity
version, the URL and the two workarounds: place a matching
base-libraries .zip in the unity-libs directory, or point the
[IL2CPP] UnityBaseLibrariesSource config at a valid URL.

Refs #1295, #1084, #986, #941, #515

Runtimes/Unity/BepInEx.Unity.IL2CPP/Il2CppInteropManager.cs

@@ -303,17 +303,34 @@ internal static partial class Il2CppInteropManager
 
             Logger.LogMessage($"Downloading unity base libraries from {source}");
             using var httpClient = new HttpClient();
-            using var zipStream = httpClient.GetStreamAsync(uri).GetAwaiter().GetResult();
+
+            Stream zipStream;
             try
             {
-                using var writeStream = new FileStream(zipFilePath, FileMode.Create, FileAccess.Write, FileShare.None);
-                zipStream.CopyTo(writeStream);
+                zipStream = httpClient.GetStreamAsync(uri).GetAwaiter().GetResult();
             }
-            catch
+            catch (Exception e)
             {
-                // Delete the incomplete file to avoid issues on next startup
-                try { File.Delete(zipFilePath); } catch { }
-                throw;
+                throw new IOException(
+                    $"Failed to download Unity base libraries for Unity {unityVersion} from {source}: {e.Message}. " +
+                    $"Base libraries for this Unity version may not be available on the server yet. Download a matching " +
+                    $"base-libraries .zip and place it in the \"{UnityBaseLibsDirectory}\" (unity-libs) directory, or set " +
+                    $"the [IL2CPP] UnityBaseLibrariesSource config option to a valid URL.", e);
+            }
+
+            using (zipStream)
+            {
+                try
+                {
+                    using var writeStream = new FileStream(zipFilePath, FileMode.Create, FileAccess.Write, FileShare.None);
+                    zipStream.CopyTo(writeStream);
+                }
+                catch
+                {
+                    // Delete the incomplete file to avoid issues on next startup
+                    try { File.Delete(zipFilePath); } catch { }
+                    throw;
+                }
             }
         }