gdb/python: convert gdbpy_err_fetch to use gdbpy_ref

gdb/python: convert gdbpy_err_fetch to use gdbpy_ref

Convert the gdbpy_err_fetch class to make use of gdbpy_ref, this
removes the need for manual reference count management, and allows the
destructor to be removed.

There should be no functional change after this commit.

I think this cleanup is worth doing on its own, however, in a later
commit I will want to copy instances of gdbpy_err_fetch, and switching
to using gdbpy_ref means that I can rely on the default copy
constructor, without having to add one that handles the reference
counts, so this is good preparation for that upcoming change.