provider: Safely access job.stderr in #Require

If `jobstart()` fails, then the subsequent `rpcrequest()` will throw due
to an invalid channel id.  This causes `job.stderr` not to exist, so we
throw another exception when trying to dump the job's stderr.

    Error detected while processing function remote#define#AutocmdBootstrap[1]..remote#host#Require[10]..provider#pythonx#Require:
    line 22:
    E716: Key not present in Dictionary: stderr

This obfuscates the actual problem.

runtime/autoload/provider/pythonx.vim

@@ -29,7 +29,7 @@ function! provider#pythonx#Require(host) abort
   catch
     echomsg v:throwpoint
     echomsg v:exception
-    for row in job.stderr
+    for row in get(job, 'stderr', [])
       echomsg row
     endfor
   endtry