[Foundation] Don't leak exceptions in WrappedNSInputStream.Read.

We don't want to leak exceptions back to the calling native code in WrappedNSInputStream.Read, because that will likely crash the process.

Example stack trace:

    ObjectDisposed_StreamClosed (System.ObjectDisposedException)
       at System.ThrowHelper.ThrowObjectDisposedException_StreamClosed(String) + 0x3c
       at System.IO.MemoryStream.Read(Byte[], Int32, Int32) + 0x124
       at System.Net.Http.MultipartContent.ContentReadStream.Read(Byte[], Int32, Int32) + 0x78
       at System.Net.Http.NSUrlSessionHandler.WrappedNSInputStream.Read(IntPtr buffer, UIntPtr len) + 0x58
       at MyApp!<BaseAddress>+0x7082f8

Instead return -1 from the Read method, which is documented as an error
condition, and then also return a custom NSError from the Error property -
which is also documented to be where the error is supposed to be surfaced.

Ref: https://developer.apple.com/documentation/foundation/nsinputstream/1411544-read

Ref: dotnet#20123.

src/Foundation/NSExceptionError.cs

@@ -0,0 +1,32 @@
+using System;
+using System.Runtime.Versioning;
+
+#nullable enable
+
+namespace Foundation {
+#if NET
+	[SupportedOSPlatform ("ios")]
+	[SupportedOSPlatform ("maccatalyst")]
+	[SupportedOSPlatform ("macos")]
+	[SupportedOSPlatform ("tvos")]
+#endif
+	public class NSExceptionError : NSError {
+		Exception exception;
+
+		public Exception Exception { get => exception; }
+
+		public NSExceptionError (Exception exception)
+			: base ((NSString) exception.GetType ().FullName, exception.HResult, GetDictionary (exception))
+		{
+			this.exception = exception;
+		}
+
+		static NSDictionary GetDictionary (Exception e)
+		{
+			var dict = new NSMutableDictionary ();
+			dict [NSError.LocalizedDescriptionKey] = (NSString) e.Message;
+			dict [NSError.LocalizedFailureReasonErrorKey] = (NSString) e.Message;
+			return dict;
+		}
+	}
+}

src/Foundation/NSUrlSessionHandler.cs

@@ -1428,6 +1428,7 @@ class WrappedNSInputStream : NSInputStream {
 			CFRunLoopSource source;
 			readonly Stream stream;
 			bool notifying;
+			NSError? error;
 
 			public WrappedNSInputStream (Stream inputStream)
 			{
@@ -1456,21 +1457,34 @@ public override void Close ()
 			[Preserve (Conditional = true)]
 			public override nint Read (IntPtr buffer, nuint len)
 			{
-				var sourceBytes = new byte [len];
-				var read = stream.Read (sourceBytes, 0, (int) len);
-				Marshal.Copy (sourceBytes, 0, buffer, (int) len);
+				try {
+					var sourceBytes = new byte [len];
+					var read = stream.Read (sourceBytes, 0, (int) len);
+					Marshal.Copy (sourceBytes, 0, buffer, (int) len);
 
-				if (notifying)
-					return read;
+					if (notifying)
+						return read;
 
-				notifying = true;
-				if (stream.CanSeek && stream.Position == stream.Length) {
-					Notify (CFStreamEventType.EndEncountered);
-					status = NSStreamStatus.AtEnd;
+					notifying = true;
+					if (stream.CanSeek && stream.Position == stream.Length) {
+						Notify (CFStreamEventType.EndEncountered);
+						status = NSStreamStatus.AtEnd;
+					}
+					notifying = false;
+
+					return read;
+				} catch (Exception e) {
+					// -1 means that the operation failed; more information about the error can be obtained with streamError.
+					error = new NSExceptionError (e);
+					return -1;
 				}
-				notifying = false;
+			}
 
-				return read;
+			[Preserve (Conditional = true)]
+			public override NSError Error {
+				get {
+					return error ?? base.Error;
+				}
 			}
 
 			[Preserve (Conditional = true)]

src/frameworks.sources

@@ -826,6 +826,7 @@ FOUNDATION_SOURCES = \
 	Foundation/NSDistributedNotificationCenter.cs \
 	Foundation/NSEnumerator_1.cs \
 	Foundation/NSErrorException.cs \
+	Foundation/NSExceptionError.cs \
 	Foundation/NSExpression.cs \
 	Foundation/NSFastEnumerationState.cs \
 	Foundation/NSFastEnumerator.cs \