BUG/MINOR: stktable: report the current proxy name in error messages

BUG/MINOR: stktable: report the current proxy name in error messages

Since commit 1b8e68e89a ("MEDIUM: stick-table: Stop handling stick-tables
as proxies."), a rule referencing the current proxy with no table leads
to the following error :

  [ALERT] 023/071924 (16479) : Proxy 'px': unable to find stick-table '(null)'.
  [ALERT] 023/071914 (16479) : Fatal errors found in configuration.

for a config like this one:

  backend px
        stick on src

This patch fixes it and should be backported as far as 2.0.