core: Fix error handling for reading from a GDataDownloadStream
core: Fix error handling for reading from a GDataDownloadStream

Cancellation should only return an error if it couldn't read a single byte.
All errors should be accompanied by a return value of -1, not 0.

Pecisk/libgdata,Pecisk/libgdata-gtasks,Distrotech/libgdata,Pecisk/libgdata-gtasks,pwithnall/libgdata,Distrotech/libgdata,Pecisk/libgdata-gtasks,Distrotech/libgdata,Pecisk/libgdata,pwithnall/libgdata,Pecisk/libgdata,GNOME/libgdata