Pass errors as Error instances rather than strings.

test/unit/metadata.js

@@ -185,4 +185,11 @@ describe('Image metadata', function() {
     });
   });
 
+  it('Report an invalid image as an error', function(done) {
+    sharp(new Buffer([0x1, 0x2, 0x3, 0x4])).metadata(function (err, metadata) {
+      assert.ok(err);
+      assert.ok(err instanceof Error);
+      done();
+    });
+  });
 });