CLEANUP: proxy/cli: take the "show errors" context definition out of the appctx

CLEANUP: proxy/cli: take the "show errors" context definition out of the appctx

This makes use of the generic command context allocation so that the
appctx doesn't have to declare a specific one anymore. The context is
created during parsing.

The code still has room for improvement, such as in the "flags" field
where bits are hard-coded, but they weren't modified.