Got lost in commit 6ed77f2a

Let std::unique_ptr manage both the InputStream and the WavpackInput.

Will replace "postponed_error".

Make all the locks exception-safe.

Code simplification.

Be exception-safe.

Prepare full C++ exception support in the decoder thread.

Prepare full C++ exception support in the player thread.

The C++ compiler will auto-generate move and copy
constructors/operators for us.

Failure must be indicated by returning false.