`provider#pythonX#Error` and `provider#pythonX#Prog` are currently meant
for debugging only (the error message is not being used), and should
therefore be defined always, especially in case of errors.
Ref: https://github.com/neovim/neovim/pull/2549#issuecomment-98607580
xsel and xcopy may be available even if a valid X display is not. Also,
the availability of X may change at any time, so check on each
invocation.
Closes#1509.
Clipboard is implemented with platform-specific shell commands, and python is
implemented with the external plugin facility (rpc#* functions). The
script_host.py file(legacy python-vim emulation plugin) was moved/adapted from
the python client repository.