noticed need for additional exception

src/AudioDecoder.cpp

@@ -117,9 +117,14 @@ std::string NyquistIO::ParsePathForExtension(const std::string & path) const
 }
 
 std::shared_ptr<BaseDecoder> NyquistIO::GetDecoderForExtension(const std::string ext)
+{
+    if (decoderTable.size())
     {
         return decoderTable[ext];
     }
+    else throw std::runtime_error("No available decoders.");
+    return nullptr;
+}
 
 void NyquistIO::BuildDecoderTable()
 {