Improve error message when capture stream fails

Rather than just guessing at the cause of the error, include the actual
error string from PortAudio.

src/AudioIO.h

@@ -729,6 +729,10 @@ private:
    volatile double     mLastRecordingOffset;
    PaError             mLastPaError;
 
+public:
+   wxString            LastPaErrorString();
+private:
+
    AudacityProject    *mOwningProject;
    wxWeakRef<MeterPanel> mInputMeter{};
    MeterPanel         *mOutputMeter;