Fixes ddiakopoulos/libnyquist#42

This aids in rethrowing exceptions from callers wanting to:
- void NyquistIO::Load(AudioData * data, const std::vector<uint8_t> & buffer)
- void NyquistIO::Load(AudioData * data, const std::string & extension, const std::vector<uint8_t> & buffer)

where std::exceptions are not raised by the catch on:
97d71768ef/src/Common.cpp (L135)

src/Common.cpp

@@ -135,6 +135,7 @@ void NyquistIO::Load(AudioData * data, const std::string & extension, const std:
         catch (const std::exception & e)
         {
             std::cerr << "caught internal loading exception: " << e.what() << std::endl;
+            throw;
         }
     }
     else throw std::runtime_error("fatal: no decoders available");