Add custom error code and message for the case when container format does not support a certain output format.

src/nvtt/Compressor.cpp

@@ -493,7 +493,7 @@ bool Compressor::Private::outputHeader(const InputOptions::Private & inputOption
 			// This container does not support the requested format.
 			if (outputOptions.errorHandler != NULL)
 			{
-				outputOptions.errorHandler->error(Error_UnsupportedFeature);
+				outputOptions.errorHandler->error(Error_UnsupportedOutputFormat);
 			}
 			
 			return false;

src/nvtt/nvtt.cpp

@@ -42,6 +42,8 @@ const char * nvtt::errorString(Error e)
 			return "Error opening file";
 		case Error_FileWrite:
 			return "Error writing through output handler";
+        case Error_UnsupportedOutputFormat:
+            return "The container file does not support the selected output format";
 	}
 	
 	return "Invalid error";

src/nvtt/nvtt.h

@@ -280,6 +280,7 @@ namespace nvtt
 		Error_CudaError,
   		Error_FileOpen,
   		Error_FileWrite,
+        Error_UnsupportedOutputFormat,
 	};
 	
 	/// Error handler.

src/nvtt/nvtt_wrapper.h

@@ -156,6 +156,7 @@ typedef enum
 	NVTT_Error_Unknown,
 	NVTT_Error_FileOpen,
 	NVTT_Error_FileWrite,
+    NVTT_Error_UnsupportedOutputFormat,
 } NvttError;
 
 typedef enum