IVGCVSW-7889 Add support for Signed64 data type in Cast layer

* Remove mention of "isDepthwise" variable name when not needed and therefore IgnoreUnused call
* Improve error messages and change them to throws in Encoder and Decoder

Signed-off-by: Teresa Charlin <teresa.charlinreyes@arm.com>
Change-Id: I8ce30b5075e1e47d54abc12390265ba8e9ee1405
diff --git a/src/backends/reference/RefLayerSupport.cpp b/src/backends/reference/RefLayerSupport.cpp
index 167639a..2be227a 100644
--- a/src/backends/reference/RefLayerSupport.cpp
+++ b/src/backends/reference/RefLayerSupport.cpp
@@ -847,7 +847,7 @@
                                       const TensorInfo& output,
                                       Optional<std::string&> reasonIfUnsupported) const
 {
-    std::array<DataType, 9> supportedInputTypes =
+    std::array<DataType, 10> supportedInputTypes =
         {
                 DataType::Float32,
                 DataType::Float16,
@@ -855,7 +855,8 @@
                 DataType::QAsymmS8,
                 DataType::QAsymmU8,
                 DataType::QSymmS16,
-                DataType::Signed32
+                DataType::Signed32,
+                DataType::Signed64
         };
 
     bool supported = true;