Unpacker.Try* throw IOException if stream ends

Previously they would sometimes return false, sometimes throw.
It's more consistent this way.

Author: drewnoakes

Dasher.Tests/DeserialiserTests.cs

@@ -189,11 +189,10 @@ public void ThrowsOnEmptyData()
             var bytes = new byte[0];
 
             var deserialiser = new Deserialiser<UserScore>();
-            var ex = Assert.Throws<DeserialisationException>(
+            var ex = Assert.Throws<IOException>(
                 () => deserialiser.Deserialise(bytes));
 
-            Assert.Equal(typeof(UserScore), ex.TargetType);
-            Assert.Equal("Data stream empty", ex.Message);
+            Assert.Equal("End of stream reached.", ex.Message);
         }
 
         [Fact]