bind: remove error wrappers to preserve error instance identity

CL 24800 changed the error representation from strings to objects.
However, since native errors types are not immediately compatible
across languages, wrapper types were introduced to bridge the gap.

This CL remove those wrappers and instead special case the error
proxy types to conform to their language error protocol.

Specifically:

 - The ObjC proxy for Go errors now extends NSError and calls
   initWithDomain to store the error message.
 - The Go proxy for ObjC NSError return the localizedDescription
    property for calls to Error.
 - The Java proxy for Go errors ow extends Exception and
   overrides getMessage() to return the error message.
 - The Go proxy for Java Exceptions returns getMessage whenever
   Error is called.

The end result is that error values behave more like normal objects
across the language boundary. In particular, instance identity is
now preserved: an error passed across the boundary and back will
result in the same instance.

There are two semantic changes that followed this change:

 - The domain for wrapped Go errors is now always "go".
   The domain wasn't useful before this CL: the domains were set to
   the package name of function or method where the error happened
   to cross the language boundary.
 - If a Go method that returns an error is implemented in ObjC, the
   implementation must now both return NO _and_ set the error result
   for the calling Go code to receive a non-nil error.
   Before this CL, because errors were always wrapped, a nil ObjC
   could be represented with a non-nil wrapper.

Change-Id: Idb415b6b13ecf79ccceb60f675059942bfc48fec
Reviewed-on: https://go-review.googlesource.com/29298
Reviewed-by: David Crawshaw <crawshaw@golang.org>

bind/java/Seq.java

@@ -9,7 +9,6 @@ import java.util.IdentityHashMap;
 import java.util.logging.Logger;
 
 import go.Universe;
-import go.error;
 
 // Seq is a sequence of machine-dependent encoded values.
 // Used by automatically generated language bindings to talk to Go.
@@ -48,18 +47,6 @@ public class Seq {
 	private Seq() {
 	}
 
-	private static void throwException(error err) throws Exception {
-		throw new Exception(err.error());
-	}
-
-	private static error wrapThrowable(final Throwable t) {
-		return new error() {
-			@Override public String error() {
-				return t.getMessage();
-			}
-		};
-	}
-
 	// ctx is an android.context.Context.
 	static native void setContext(java.lang.Object ctx);
 
@@ -105,23 +92,10 @@ public class Seq {
 		// invalidating it before being translated on the Go side.
 		int incRefnum();
 	}
-
 	// A Proxy is a Java object that proxies a Go object. Proxies, unlike
 	// GoObjects, are unwrapped to their Go counterpart when deserialized
 	// in Go.
-	public static abstract class Proxy implements GoObject {
-		private final Ref ref;
-
-		protected Proxy(Ref ref) {
-			this.ref = ref;
-		}
-
-		@Override public final int incRefnum() {
-			int refnum = ref.refnum;
-			Seq.incGoRef(refnum);
-			return refnum;
-		}
-	}
+	public interface Proxy extends GoObject {}
 
 	// A Ref is an object tagged with an integer for passing back and
 	// forth across the language boundary.