provider/python: define Prog and Error functions always

`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

runtime/autoload/provider/python.vim

@@ -10,10 +10,6 @@ endif
 let g:loaded_python_provider = 1
 
 let [s:prog, s:err] = provider#pythonx#Detect(2)
-if s:prog == ''
-  " Detection failed
-  finish
-endif
 
 function! provider#python#Prog()
   return s:prog
@@ -23,6 +19,11 @@ function! provider#python#Error()
   return s:err
 endfunction
 
+if s:prog == ''
+  " Detection failed
+  finish
+endif
+
 let s:plugin_path = expand('<sfile>:p:h').'/script_host.py'
 
 " The Python provider plugin will run in a separate instance of the Python