If setting a system-variable provided by a plug-in failed, no OK or
error was sent in some cases, hanging the client. We now send an error
in the case from the ticket (integer-argument out of range in STRICT
mode). We also provide a semi-generic fallback message for possible
future cases like this where an error is signalled, but no message is
sent to the client. The error/warning handling is unified so it's the
same again for variables provided by plugins and those in the server
proper.


mysql-test/r/plugin.result:
  show that on out-of-range values, plugin interface throws errors
  in STRICT mode and warnings otherwise.
mysql-test/t/plugin.test:
  show that on out-of-range values, plugin interface throws errors
  in STRICT mode and warnings otherwise.
sql/set_var.cc:
  - handle signedness of values used in warnings
  - in STRICT mode, throw errors rather than warnings
sql/sql_parse.cc:
  If sql_set_variables() returns with an error but no message
  was sent to the client, send a semi-generic one so the session
  won't hang and we won't fail silently.
sql/sql_plugin.cc:
  throw a warning if more than just block-size was corrected
  (or an error in STRICT mode). use functions from set_var
  for uniform behaviour of server- and plug-in variables.
storage/example/ha_example.cc:
  Add a ULONG system variable to example plugin so
  we can test integers in the plugin-interface without
  having to depend on the presence of innobase.